while it is possible to unlock a jailbroken iphone 3gs using the 06.15.00 baseband upgrade, it comes with a set of risks. one of those being -- it's possible to get stuck in the dreaded recovery loop (stuck on the apple logo or low battery warning and restarting after a few minutes). it's a tethered boot so if you reset the phone without it being tethered, or if it crashes, your phone may be temporarily bricked. iphones can never be truly "bricked" by software means, but you may be limited in what version of the ios software you may run. i had successfully unlocked the 3gs and used it internationally, but after i got back to america the phone was stuck in a loop after turning the phone off.
i had to sift through many, many blog posts and forum discussions of how to get the iphone 3gs in a usable state after getting into a recovery loop after a baseband update. at first i tried to use redsn0w to simply jailbreak it again, no dice. it would "jailbreak" but then would go back into the loop. i tried to restore with itunes -- won't work because of the changed baseband. i tried to use a custom 4.2.1 firmware to restore with itunes -- won't work either. then i put the iphone into dfu mode and fired up tiny umbrella to try to kick it out of recovery mode. it worked, but it got stuck on a greenpois0n init screen.
after a few hours of frustration and trying to restore over and over in different ways, i finally found a method to get the iphone working again. i found a forum post that mentioned that apple was still signing the 4.1 ipsw and does not require a shsh blob. i put the iphone in dfu mode, then fired up itunes then tried to restore to the 4.1 firmware. it actually started to install, but it would eventually pop up a 1015 restore error. after that however, i opened up tiny umbrella and clicked on the iphone on the device list. in advanced, i unchecked "set hosts to cydia" and then clicked "fix recovery." then it would load up the greenpois0n init screen, but once i restarted the iphone, it booted into 4.1 without any problems and i was able to use the phone as normal (with an at&t sim). despite being stuck on 4.1 for now, it's at least better than a non-working iphone 3gs.
it's also another constant reminder to always back up your data -- once your iphone gets stuck in recovery you'll lose all your data through the restore to get it working again.