As expected, that was fast. The current exploit used by jailbreaking tools greenpois0n, redsn0w and limera1n attacks the iOS device's boot rom, which is outside of the bits that Apple can change in software. As such, it cannot be addressed without issuing new hardware, meaning that, for the time being, all iOS devices are capable of being jailbroken using the same method.
Our own Erica Sadun reported iffy results from an early jailbreak of a dev build of iOS 4.2, but these have been resolved, and iPhone Dev Team are reporting that their redsn0w tool can now jailbreak all iOS 4.2.1 devices.
There are some caveats, however.
Firstly, if you use the ultrasn0w tool to unlock your phone (so you can use it with any carrier), then you have two problems. One, the tool itself doesn't work on iOS 4.2; two, if you allow iTunes to update to the newer versions of the baseband software in 4.2, then it might never work again on your phone. If you rely on carrier unlock, or if you think you might ever come to rely on it, you should always stay away from iOS updates until one of the dev teams supplies a tool to create custom IPSW files. This will give your phone all the creamy goodness of 4.2.1 but keep the old, unlockable baseband software in place.
Secondly, on newer devices (specifically iPhone 4, newer model iPhone 3GS, iPod touch 3G and beyond, and iPad) this is a so-called tethered jailbreak. This means that whenever you reboot your device, you have to physically have it connected to your Mac or PC with the jailbreak software running; otherwise, it'll boot back up without the jailbreak in place. Obviously this can be a serious nuisance.
Expect both of these caveats to be addressed in time, although there is , of course, no guarantees as to when. If you are not put off, then Redmond Pie have a really nice walkthrough of the jailbreaking process.
Thanks to everyone who sent this in.