A bit late...Pex 0.6 (with x64 support) released
I've not looked at Pex (Program EXploration) yet but it seems pretty interesting. The previous version 0.5 only supported x86.
I watched the Channel9 video on Pex a while back now, and have been following it via word of mouth for a long time so when 0.5 dropped it was pretty exciting...until I found out it didn't support x64.
Nikolai Tillman has some more information on the 0.6 release, of particular note "64-bit support: Pex 0.6 installs on 64-bit Windows (but cannot analyse 64-bit-only code)"