This one's definitely not for the casual iPhone unlocker, but if for some reason you're looking to toy around with a couple of different firmware versions at a time, you can now apparently set up a dual-boot system of sorts on the phone. As Jonathan "NerveGas" Zdziarski notes in his how-to, the hack in question has actually been in use by the iPhone Dev Team for "several months" now, but he says that "now that several more advanced techniques have been developed," they've decided to let it loose for all to use. Needless to say, there's some partition-making and a good deal of command line work involved in the feat (all of which could cause you some serious headaches), but once you've got everything set up, you'll apparently be able to switch between different OS versions just by changing your root-device with a simple command. As iPhone Atlas points out, that option will likely become a lot more attractive once the SDK-compliant 2.0 firmware is released, but if you've got some ideas of your own in the meantime, you can find all the details you need at the link below.
[Via iPhone Atlas, thanks Shan]