This is with regard to questions on vacancy rates by season. 

I recently had a chat with Jon Sperling at HUD (during a walk on the mall) and he pointed me to these datasets (acquired from USPS) that list vacancy rates for each census tract by quarter and year.

http://www.huduser.org/datasets/usps.html

I have not used these much, but am intrigued.  The datasets are comparatively new (introduced in December 2006).  HUD is looking for user feedback.  I found that the data can be joined and easily plotted using a nationwide Census Tract shapefile.  I can see quite a few applications in examining these data for transportation planning applications.

Also see the data dictionary at http://www.huduser.org/datasets/usps/USPS_Data_Dictionary.pdf

The contact person for these datasets is Robert Renner.

Thank you

Nanda Srinivasan