I thought you could tether from Blackberry Curve...? A quick google search seems to show lots of people doing it. Like this:
http://forums.crackberry.com/f...0-tethered-modem-2038/
You can tether from the iPhone right now - there's an app store app called NetShare that was pulled and reinstated and then pulled and reinstated, and so on, and a bunch of people are using it. But you can't, to the best of my knowledge, buy it now. Or you can run QuickPwn on your iPhone, and then install PDANet using the Cydia installer. Netshare and PDANet both tether wirelessly by setting up the iPhone as an ad-hoc network - and then you have your laptop connect to that ad-hoc network for connecting to the internet. PDAnet can also tether through USB if you don't want to muck with ad-hoc networks.
All 4 major US carriers - and plenty of the minor ones like Cricket - have "unlimited" data plans, and most recent smartphones can be made to tether. Blackberry, Windows Mobile, Symbian - they can all tether.
So then there's packages and who allows it and who doesn't and how they can tell you are doing it. So, the problem with tethering is that the wireless guys are really worried about people cancelling their DSL connection and then firing up Bittorrent on their tethered modem. So they don't encourage the practice and price it high to make it unappealing. They can definitely tell that you are tethering if they want to - browsers all send out their OS and build info and if AT&T sees HTML traffic originating from Internet Explorer for WinXP, they'll have no trouble concluding that it's not coming from someone's Blackberry Bold. People who tether also tend to use a LOT more data than people on cell phones - so just sorting users by data traffic usually is a great way to identify tethered users. And carriers will definitely contact you - or drop you - if you move too much data around.
That said, if you stay reasonably under the radar, you are likely to be fine. I've tethered my iPhone while travelling and it's been ok (if slow, cause it's on EDGE) and no one has ever contacted me about it. I would presume that if you got the data package from AT&T and tethered your iPhone and didn't use it excessively, you'd be fine.
Or you can get their tethered plans for their phones - which are expensive but then you don't have to worry about getting a call from a customer service rep. They usually have bandwidth caps - like 5GB. Not all phones offer tethered plans - the iPhone in particular does not right now - although there's rumors of a plan coming this summer.