[ame=http://www.vimeo.com/5431060]iPhone 3GS Unlock Demonstration on Vimeo[/ame]
Lets keep this short and sweet. Yes, iPhone 3GS can be unlocked. No you can't yet (jailbreak and what not isn't released).
Until then the devs say DO NOT UPGRADE to firmware 3.1 if you want to be sure you can unlock when the 3GS jailbreak is released.