If you head on over to the Windows Dev Center, you can download the Developer Preview version of Windows 8 (Client) in 32-bit and 64-bit flavours.
If you have an MSDN subscription, you will find additional ISOs for the Server version and goodies like the development kits and symbols packages too.
Over at http://www.buildwindows.com/ you can watch the keynote from the BUILD conference for a great look at the new features coming up.
ZDNet have an article going into enthusiastic detail on the features in the Server version, which is worth a read.
It’s definitely looking like the Metro interface and heavy use of PowerShell are going to be major game-changers for software developers from here on…