I agree with the OP. Looking for eg next week, 24-25May08, in Four Points by Sheraton Panoramahaus Dornbirn, Austria, the hotel list promises rates from EUR85 but "Book now" gives rates as from EUR110. This has been the case consistently in the recent weeks.
If I search for hotels for N people, for dates D1-D2, I think I can expect an online reservation system to return rates that match my search criteria. The fact that this hotel might have lower rates for criteria different from mine is in this context misleading.
I would accept the display of these lower fares, however, if spg.com had a feature "show me when/how these fares are really available".