I switched to Straight Talk on an iPhone 4 (AT&T GSM) right after my contract expired, with basically no problems. I'm a light data user (web/twitter/email/maps, no streaming music/video), so $45/month is a decent bargain for "unlimited" everything.
Now, if there was only a decent prepaid data-only plan for tethering. What I want is to buy a few gigs of transfer that doesn't expire (for at least a few months) and is tied to a USB stick. Nobody appears to sell this.
Also, I do tether it for my daily train commute, but please don't tell them that.
Googling for things like "iPhone ATT prepaid" didn't yield good results. The power of a human recommendation is infinitely useful. And backed up by an Ars technica article is double win.
This is how they luck us in, even though we'd truly prefer to be somewhere else.