I'm just going to throw this out there as an AT&T user with grandfathered $30 unlimited data. In the past 6 months, I've used an iPhone 4, HTC Aria, Dell Venue, and Nexus S as my primary phone. Obviously, the iPhone 4 required jailbreaking to tether without a supplemental fee. The Aria was AT&T-branded, but I installed CM7 which included the stock tethering settings menu. The Dell Venue and Nexus S are sold unlocked. The Venue was purchased directly from Dell, and the Nexus is a Canadian variant that I bought from Negri for the 850MHz HSDPA support. Both these non-AT&T branded phones allow open use of the stock tethering capability in Android.

I don't tether much, but I like to have the option in an emergency. Paying full-price for a phone upfront is worth the freedom to me. Naturally, unlocked phones also sell for more money on ebay when I'm done with them. I've actually managed to make a healthy profit unlocking Android phones, installing CM7 and flipping them on ebay while I was working-out which phone to settle on.

FWIW, I settled on the Nexus S. I haven't even rooted it. I'm just that content with it.

I defected from the iPhone primarily for Android's tight integration with Google Voice. Now that I've successfully transitioned my contacts to dial my GV number, I'm free to dump AT&T and move to some other carrier without the worry of number porting. I'm currently still under contract with AT&T, but I plan to make a move to either Straight Talk (AT&T MVNO) or Simple Mobile (T-Mobile MVNO). Both have unlimited data at a reasonable price, and I doubt they will care if I'm tethering a few times per year.