Latest iPhone 3GS’ shipped by Apple seem to be immune to jailbreaking
It looks like Apple is finally trying to put an end to jailbreaking the iPhone.
Reportedly, all the iPhone 3GS units shipped starting last week have a new botroom (iBoot-359.3.2) that makes jailbreaking impossible.
According to iClarified, iBoot-359.3.2 is not vulnerable to the 24kpwn exploit which is used by the Dev-Team (among others) to jailbreak older iPhones and iPods.
So if you’re getting a new iPhone 3GS and want to jailbreak it, you should look for old stock, or buy a refurbished unit.
The new iPhone bootrom may have other exploits than the 24kpwn, but it will probably take a while until developers find them.