Geohot released blacksn0w tool to unlock iPhone 3G or 3GS with baseband 05.11.07. First you need to jailbreak iPhone with blackra1n and then you’ll be able to work with unlock activity through blacksn0w.

According to Geohot:

blackra1n RC3 is also live for both Windows and Mac!!! Clean firmware 3.1.2 and baseband 05.11.07 are recommended.
Updates in BlackRa1n RC3:
Hacktivation
Unlock Baseband 05.11.07
Keep legit activation if activated before running app
15 seconds faster, you’ll feel it
Tiger + PPC support

Downlaod Blacksn0w for Mac or Windows