Nathan Erlbaum of NY State DOT has provided some links that may help you when the first small area data are released from the ACS (2005-2009). 

Note:  the smallest geographic unit will be census tract (2000 geography).  Block Group data will be released later in Summary File format.

From: Erlbaum, Nathan (DOT) [mailto:nerlbaum@dot.state.ny.us]
Sent: Tuesday, December 07, 2010 5:01 AM
Subject: FW: ACS 2005-09 comparisons with 2000 SF3

FYI.  This may help next week when the 5-year ACS data are released.

It appears that the Census Bureau has updated its guidelines for comparing the 5-year 2005-09 ACS data to 2000 data.  When I checked this link today (http://www.census.gov/acs/www/guidance_for_data_users/comparing_2009/), I noticed that it included the 5-year data, which had been missing when I last checked just a week or so ago.

Also, the Bureau provides a variable-specific drill-down tool here: http://www.census.gov/acs/www/guidance_for_data_users/2009_table_comparisons/   Pretty neat.