The Dev-Team have already developed a jailbreak for the iPhone 3GS but they're holding it back for now as they expect Apple to release an update soon to fix issues in the current 3.0 firmware.
Here’s the critical point, the reason why we’re delaying our version of the jailbreak: Once the jailbreak is out, Apple will fix the iBoot-family bug we use to accomplish it. They will simply stop signing the old iBoots and only sign the fixed ones. If you bought your phone after Apple has done this, there’s nothing you can do…the jailbreak isn’t going to work for you.It is possible that Apple will find the bug we use without our handing it to them on a silver platter (via a public jailbreak). In that case, we will have delayed our jailbreak for “nothing”. But we’d rather be safe than sorry!Apple is surely coming out with a 3.0.1 firmware release shortly. They need to fix ultrasn0w. They need to fix some UI issues. 3.0 is buggy and 3.0.1 is coming. We’re going to wait and see what 3.0.1 brings before figuring out the release date for our version of the jailbreak.
What it basically means is that we'll get it soon. If you're thinking about getting one overseas then it's a good time to go hunting seeing as the phone will probably take 6-10 working days to get to you.