if you foresee yourself using the ipod function a lot, talking a lot, using wifi a lot, etc... then i highly suggest you get an auto charger. I can typically make it through a whole day on one charge, but sometimes it's close, so i got one just to be safe. And i'm sure there are tons of people out there that are more demanding of their iPhones than i am.
as far as protection goes, that really depends on you. Mine stays in my pocket, and it is completely scratch free. Although if you know you are prone to dropping your phones, then i would suggest you look for something. Cases are highly personal things, so i'll just point you here:
my next accessory purchase is going to be a set of replacement headphones that have the built-in clicker/mic. I can't recommend any, because i don't have any yet (obviously), but here is another thread with a pretty comprehensive list:
i would stick to either of the chargers on Apple's website (XtremeMac and Griffin). Apparently they have very specific specifications before an accessory can be labeled with the "works with iPhone" badge. Anyways, as i was reading the reviews of the two chargers i read equal amounts of reports that EACH charger would sometimes overheat iPhones and sometimes even lead to blown fuses and smoking within the car's panel. So i decided to risk it and went to the Apple store and bought the extrememac charger (it looks cooler). After being very careful with it for over a week i have experienced none of these reported issues. The only problem i have had is that the plug is somewhat difficult to remove from the iPhone.
I certainly don't believe so many reports are bogus, so i'm thinking it really depends on what car you are using. So i recommend you purchase one at the Apple store (not over the internet), go right to your car and try it out. from what i hear, if you are going to have problems, you will have them right away. So when you first plug it in, keep checking your phone and charger for any over heating, and sniff for any unusual burning scents. If you do run into any of these, you won't have to make a trip all the way back there to exchange/return it. In any case, the return policy is 14 days.