Microsoft has just released an updated version of Windows Vista to technical beta testers. Build 5728 is available in either x86 or x64 flavors right now on Microsoft Connect.
x86
vista_5728.16387.060917-1430_x86fre_client-LRMCFRE_EN_DVD.iso, 2,622.25 MB
x64
vista_5728.16387.060917-1430_x64fre_client-LRMCxFRE_EN_DVD.iso, 3,628.64 MB