Nice summary. I'm paying $64/month to AT&T right now (450 minute plan plus data with a FAN discount) for my iPhone 3GS. I'm seriously considering switching to Straight Talk. There are 2 reasons I'm delaying:
1. I'll probably be ready to upgrade on the next iPhone release.
2. I still have an unlimited data plan on AT&T. I leave AT&T and there's no getting that back.
With those two things, it's a better deal to stay on AT&T. The big reason ST is so cheap is that it doesn't have a device subsidy.