Apologies for the delay - meant to blog about this (and lots of other stuff) much earlier in the month. We released a new Community Tech Preview of Windows HPC Server 2008 this month. The good news is that you can join the HPC beta today, download an ISO of an evaluation copy of Windows Server 2008 and then install the CTP on top and start to explore the world of High Performance Computing.
Simply head over to http://connect.microsoft.com/directory/ and you should see HPC listed (select "Windows" and it is the last entry)
NB: You will need to have access to an x64 machine and a windows live login.