Slashdot story about a New Phoenix BIOS Starts Windows 7 Boot In 1 Second. Technically this has nothing to do with Windows 7, but more about the BIOS getting to the point where it can start booting the OS, whatever it is. I think that this type of BIOS (EFI) can boot MacOS on Intel, so this would really apply to whatever OS you want (though MS has definitely optimized their WIn7 bootup).

That said, I'm not sure how really relevant this is anymore. When was the last time you booted a machine? My windows server at work stays on 24/7, my MacBook just gets it's lid closed when it's not being used, and my Linux server is... well, a server. All my machines really only are rebooted when they have software updates that require it, so reboots are pretty few and far between. Not that I wouldn't mind a 20 second boot when it does happen of course :)