Jailbreaking iPhones is something the Dev-Team and others have been able to pull off with every new hardware and software release from Apple. That might halt, at least temporarily, with new jailbreak-resistant firmwares included in the newest iPhone 3GS models.
The Dev-Team and other jailbreak makers have been using an exploit known as 24kpwn to break into the iPhone and obtain deeper access to install new app markets and unlock certain features. A new booting firmware update, iBoot-359.3.2, has been shipped on 3GS models manufactured since last week however, and a Dev-Team member tells the Boy Genius Report that jailbreaking won't be possible on the newer units, at least until another exploit is found.
It's odd to see Apple spend so much effort on fighting back against a niche group of iPhone enthusiasts, but then again, AT&T has an interest in keeping things like tethering to a minimum. For the time being, if you're looking to break into a newer iPhone model, hold off until another exploit is (inevitably) found.