The iOS 4.3 Gold Master has already been jailbroken, meaning you can do cool things like enable the four and five finger gesture support on your iPhone 4. Just follow these instructions and these, for multitouch.
Warning: This jailbreak is a tethered one, meaning you have to manually boot it into a jailbreak state every time you reboot your phone. That's annoying, so don't do this unless you're really impatient. [Redmondpie]