For a nice article on ACS sampling, please see:
http://www.esri.com/news/arcuser/0408/samplesize.html
Sample Size Matters: Caveats for users of ACS tabulations by Lynn
Wombold, ESRI Chief Demographer
ArcUser Winter 2008 pps 9-11
Elaine Murakami
FHWA Office of Planning
206-220-4460