The iPhone Dev Team had promised a jailbreak for the iPad "soon" after the 3G variant's release, and they weren't kidding around. Spirit is an untethered jailbreak - i.e. you don't need to regularly plug the iPad (or indeed any iPhone or iPod touch running 3.1.2, 3.1.3 or 3.2) into your computer to maintain the hack - and takes advantage of an existing security hole in iPhone OS.
The nature of that hole means that the iPhone Dev Team expect Apple to move to patch it sooner rather than later. Their advice is to take a few steps to backup your device's current details - instructions are here - so that even if Apple do release a patch, you should still be able to jailbreak your device.
Spirit works on both the iPad WiFi and the iPad WiFi + 3G, and will allow you to install the Cydia unofficial app store onto the tablet. It's worth noting that it's not a carrier unlock for iPhone models.
Unboxing iPad WiFi + 3G:
[via Everything iPad]