No expert on real estate investment, however as far as I can tell, there are a few more factors that enter into the equation. Many have mentioned that the rental market is softening, as a result of major incentives to first time home buyers with minimal required downpayments and no income verification. Now, the owners/ landlord's of rental properties have growing supply, they are forced to make concessions they did not have to make 3-4 years ago. Maybe they include a free month of rent, perhaps they include gas and electric.
The problem, as I see it, is that while the landlord's are getting squeezed by the prospective tenant, they are also getting squeezed by the local government(in the form of higher assessed values=higher property taxes), higher insurance premiums, higher natural gas prices(maybe electricity prices as well). So not only does the P/E ratio for real estate diverge on a basic analysis of rental income/potential sales price, but the additional overhead continues to squeeze him indefinitely.