The market this summer has been driven by the first-time buyer tax credit buyers and investors buying foreclosures. Falling apartment rents will force housing landlords to drop their rents, thereby reducing the price that an "investor/speculator" will be willing to pay for investment properties.
As for the tax credit buyers, it is very likely that most first time buyers who were considering buying a home have already taken advantage of the tax incentive (similar to the cash for clunkers program, in which the majority of the sales occurred early on in the program). This dynamic is where the superficial analysts and headline skimmers believe that the housing market has "bottomed." So,even if Congress extends the taxpayer subsidization of home sales, it is likely that sales from the use of the tax credit will quickly decline.