I've been using this for nearly three months with no issues at all. There was one month that I used over 10gb of data! I know, I'm asking for it, but I have not been charged extra. The reason I was tethering so much was because I was having some issues at home with my network. Now I only use it for my netbook when I'm out and about. Works great!
I have a friend that works for AT&T and he explained how these tethering solutions work, and how you'll "maybe" get caught.
Most of these "prohibited" tethering solutions (for iPhone, Symbian, Android, and Windows Mobile) are not readily detectable by your provider. They can't realistically examine every packet from every user, nor can they "see" your devices and find out what you're running on them. What they can and do is look for spikes in usage (both data and voice minutes - for different reasons) and if they see a spike they "may" investigate further. So if you've had an iPhone for 6 months, and average 200mb a month using Safari, Mail, YouTube, etc, and then your usage suddenly spikes to 1.5GB in one month. They flag your account for a closer look see. Then they can start some network packet captures from your connection, if they find packets related to known non-iphone applications (bittorrent, IE, Google Chrome, etc) then they can and sometimes will send you a hell of a bill.
Sometimes they just watch your account for a few months and see if it goes down to a normal level - particularly businesss accounts - lots more to lose if you upset a company with 1 bad Apple out of 200 accounts.
So the rule I follow - and have used these apps on all my platforms, is use it sparingly, try to stick to web browsing (you "could" claim a Safari spoofing of IE for instance) and other reasonable and light data amounts. Not all users who use 10gb in a month will get caught and charged, but those are much more likely to be, than the occasional netbook web browser tethered to his iPhone is.
Oh sure they do, for many of their phones - smart and normal. But they can be expensive and/or like in the case of the iPhone they don't "yet" offer one.
They also wouldn't make sense for many users because they may only need it here and there - on a trip somewhere, or if your home connection goes down, etc. Hard to justify $40-80 per month if you're don't need it quite a lot, and if it's work related many companies will pay for it for their employees.
People just like to not have to pay the extra charges if they don't have to. Many get away with (as have I) but a few do get caught.
I was just pointing out what I've been told about how people get caught when they're not paying for it. YMMV
Well I've used the free hack that enables Bluetooth and USB tethering and the BT connection worked for my laptop. My iTunes install (required for the USB connection) is on my desktop so I haven't tried that one.
I actually bought MyWi ($9.99) from Rock Apps because wifi connections are a little faster, and it will work with any system (like a friend's laptop for instance) really easily as opposed to having to pair the phone with the laptop through Bluetooth.
So again, if you keep your usage reasonable, and stay away from things like HD video streaming, or P2P apps, etc, you should be ok.