AT&T Nexus S: is this sold SIM locked / boot-loader locked? I know this is a bit localized, but I can't find ANY information on this at all on the internet, because all of the information for the Nexus S assumes the T-Mobile band version.

I would only purchase this phone to immediately unlock the bootloader for custom ROM development.

Is the only difference between the two phones the bands the 3G radio uses?
If the bootloader comes locked, it can be easily unlocked.  Google doesn't let the flagship devices be sold any other way, since they are designed for development.

Endgadget says it's SIM locked. This review says it's unlocked, but I bet they didn't buy it with a contract.

The specs for Best Buy's "T-Mobile version" state that it works for EDGE (2.5G) on the 850 and 1900 MHz frequency bands, in addition to others, which are what AT&amp;T uses.  So EDGE should be usable on any GSM network in the world (I can't find the specs to see if the AT&amp;T version is likewise compatible).  3G/HSPA won't work on AT&amp;T and some others, though, since it's limited to the 900/1700/2100 bands used by T-Mobile.  Google confirms this info.