<<Disclaimer: Verify this information before applying it to your situation.>>

Bob's Red Mill:

Definite cross-contamination. They grind everything on the same mill,
so unless they completely separate their gluten/GF grains, this is an
ongoing problem.  On their rice flour, (and maybe the cornmeal?) they
use the first batch of ground grains for livestock feed, so the gluten
concentration isn't as high in those products as in their amaranth
flour, etc. But its still there! So find a source you can trust...
        [Thanks to the many respondents that answered this same question
for me a while back!]

Gluten Free Pantry:

Much better option. See the FAQ or celiac web page for other places, too.
(Like Ener-G, etc.) One of these:

http://www.fastlane.net/homepages/thodge/archive.htm
http://www.panix.com/~donwiss/

Ze