In a recent MSDN XNA Developer Center Release Notes document Microsoft stipulated that SP1 would be required for their most recent preview:

The August 2007 Direct3D 10.1 Tech Preview requires the Windows Vista SP1 Beta which will be available to MSDN subscribers once it is publicly released.

The next preview isn’t due until November. One would think that since SP1 is required for the preview, they wouldn’t release it unless SP1 was imminent. If SP1 wasn’t due until November, as some rumors have suggested, you’d think they would have held this preview until then as well. So, if you’re placing bets on when to expect SP1, here’s one more hint that it may be sooner rather than later (for the beta at least).