Found most of these in the Apple forums, but some have been posted on iLounge and Macrumor forums as well as here. 
Compiled iPhone Updates - Stuff Apple Didn't tell you
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1. Many people are reporting that the Volume of the Speakerphone and Earphone has been improved (louder).
However, I think this is a better explanation:
I got an iPhone on day 1 and thought the volume was low. I did a restore a week or so later and the volume was noticeably louder. I think the phones came out of the Factory with the volume stuck at a lower max setting, and that restoring (or updating firmware) is resetting it to the correct level. If you didn't do a restore before now, the firmware update restored the intended louder volume. BTW, a few times I thought the volume was low until I realized my lower right thumb was covering the tiny speaker when I hold the phone in landscape. I think this might be happing to others, too.
2. Before, my "junk" folder in mail.app (imap folder) didn't show up, only the Inbox, Drafts, Sent, and Trash did. Now they ALL show up properly in the iPhone! Occasionally, I get a junk email that gets filtered from my mail server/mail.app junk filter into the junk folder that's a proper email. Now I have the option to check it.
Also a very nice touch ... emails in the junk folder do not show up as "unread" messages in mobileMail. So you don't get mail badges that include the junk emails.
3. I noticed that now when you visit a web page using flash, you no longer get the little blue cube, but rather an option to download the plug-in. Problem is, that's as far you can get. No way I see to download and install the plug-in.
4. The VPN settings now correctly remember your VPN password, and there is also now a button to terminate an attempt to connect to a VPN if it hangs. It now saves the alphanumeric password, no more using numbered passwords.
5. Before, when you launched calculator and started tapping numbers, it would concatenate (link together) to whatever number was already on the screen. Now your new entry replaces... Which it should!
6. I noticed a fix in Google Maps. Before, if you stretched your fingers too far and touched the url area, the map would shoot off miles away from center. That doesn't happen any more.
7. In settings; mail; they fixed it so that on send, we can bcc instead of having to cc ourselves.
8. I have an Airport wireless router, and I had it configured to be a "closed" network, i.e., one where you could not see the network name. With 1.0, I could join the network, but if I turned the phone off, or was out of range, I had to retype the network name and password.
With 1.0.1, it now remembers the network, and automatically rejoins it, and the network is now a "closed" one.
9. For the first time since i bought the iPhone on the 29th, I get a full battery reading on the meter, and it glows! I have never seen this before, perhaps another one of the hidden updates?
10. PASSCODE LOCK now offers 5 min; 10 min; 15 min; 30 min; or 1 hour options to request the PASSWORD where before it was only IMMEDIATELY or 1 minute.
11. Usage stats reset properly after full charge with the wall charger (to see description of the bug I and others were experiencing, see http://discussions.apple.com/message.jspa?messageID=5036884#5036884)
12. You will all notice that the Most Viewed in YouTube is now different for All Time/This Week/Today.
13. iCarPlay from Monster and iHome are now also working (after saying no to the airplane mode warning) after the update. It previously did not work.
If you've found some not listed above, please add them.
Compiled iPhone Updates - Stuff Apple Didn't tell you
-----------------------------------------------------
1. Many people are reporting that the Volume of the Speakerphone and Earphone has been improved (louder).
However, I think this is a better explanation:
I got an iPhone on day 1 and thought the volume was low. I did a restore a week or so later and the volume was noticeably louder. I think the phones came out of the Factory with the volume stuck at a lower max setting, and that restoring (or updating firmware) is resetting it to the correct level. If you didn't do a restore before now, the firmware update restored the intended louder volume. BTW, a few times I thought the volume was low until I realized my lower right thumb was covering the tiny speaker when I hold the phone in landscape. I think this might be happing to others, too.
2. Before, my "junk" folder in mail.app (imap folder) didn't show up, only the Inbox, Drafts, Sent, and Trash did. Now they ALL show up properly in the iPhone! Occasionally, I get a junk email that gets filtered from my mail server/mail.app junk filter into the junk folder that's a proper email. Now I have the option to check it.
Also a very nice touch ... emails in the junk folder do not show up as "unread" messages in mobileMail. So you don't get mail badges that include the junk emails.
3. I noticed that now when you visit a web page using flash, you no longer get the little blue cube, but rather an option to download the plug-in. Problem is, that's as far you can get. No way I see to download and install the plug-in.
4. The VPN settings now correctly remember your VPN password, and there is also now a button to terminate an attempt to connect to a VPN if it hangs. It now saves the alphanumeric password, no more using numbered passwords.
5. Before, when you launched calculator and started tapping numbers, it would concatenate (link together) to whatever number was already on the screen. Now your new entry replaces... Which it should!
6. I noticed a fix in Google Maps. Before, if you stretched your fingers too far and touched the url area, the map would shoot off miles away from center. That doesn't happen any more.
7. In settings; mail; they fixed it so that on send, we can bcc instead of having to cc ourselves.
8. I have an Airport wireless router, and I had it configured to be a "closed" network, i.e., one where you could not see the network name. With 1.0, I could join the network, but if I turned the phone off, or was out of range, I had to retype the network name and password.
With 1.0.1, it now remembers the network, and automatically rejoins it, and the network is now a "closed" one.
9. For the first time since i bought the iPhone on the 29th, I get a full battery reading on the meter, and it glows! I have never seen this before, perhaps another one of the hidden updates?
10. PASSCODE LOCK now offers 5 min; 10 min; 15 min; 30 min; or 1 hour options to request the PASSWORD where before it was only IMMEDIATELY or 1 minute.
11. Usage stats reset properly after full charge with the wall charger (to see description of the bug I and others were experiencing, see http://discussions.apple.com/message.jspa?messageID=5036884#5036884)
12. You will all notice that the Most Viewed in YouTube is now different for All Time/This Week/Today.
13. iCarPlay from Monster and iHome are now also working (after saying no to the airplane mode warning) after the update. It previously did not work.
If you've found some not listed above, please add them.
