The iPhone Dev-Team has posted a few words regarding Jailbreaking the 3.0 software:
Because there are so many beta releases, we couldn’t possible refine, test, and release both PwnageTool and QuickPwn for each of them. That’s why we’re waiting until the final release. You may have seen other “hijacked” versions of QuickPwn out there, but all of them are buggy, none of them work on OSX, and almost everyone who uses them reverts back to 2.2.1 (because none of the useful jailbroken apps (Qik, Cycorder, and others) work on 3.0 yet).
Now this does not mean you currently can not Jailbreak 3.0 because you can, it's just that the Dev-Teams software is not fine-tuned for the iPhone beta software. If you do decide to Jailbreak the 3.0 beta you will run into some issues. The biggest issue, as stated above, being a lot of the applications available via Cydia will not be compatible with 3.0. It is also important to note that if you ever want to unlock your iPhone 3G, you need to avoid the 3.0 software altogether unless and until the Dev-Team provides a safe jailbreak path.
So our advice to the Jailbreakers of the world: have some patience. When the final build of 3.0 drops sometime this summer you will be able to Jailbreak your iPhone 3G.
(The only way Apple could prevent a Jailbreak does not rest within the software, rather the hardware. As for Jailbreaking the next generation of iPhone, only time will tell, we are sure Apple will make it interesting...)