AT&T Nexus S: is this sold SIM locked / boot-loader locked?
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.