Here's a link to a page with a lot of
information about bicycle helmet testing standards:
http://www.helmets.org/standard.htm
Relative to the most recent discussion on this thread, that page explains about four different standards used for testing bike helmets, and notes that the standards are similar to each other in gauging the amount of impact helmets can absorb. Among ways the standards differ, is in requiring slightly greater head coverage (Snell), and in the way test labs get test samples.
Manufacturers that would rather not pay Snell's price for certification, can have their helmet designs meet the CPSC standard, whatever the cost for the latter certification is. The page that I provided a link to doesn't mention penalties in the U.S. for not meeting at least the CPSC standard. Untested helmets wouldn't likely be greeted warmly by consumer watchdogs.