Sallie Goetsch explains the basics of setting up WordPress Multisite, Multisite domain mapping, and shows examples of a couple of small networks at the July 2012 East Bay WordPress Meetup.
25. Extra
# Next two lines fix Multi-site wildcard problem
RewriteCond %{HTTP_HOST} ^rhymeswithsketch.com$ [NC]
RewriteRule ^(.*)$ http://www.rhymeswithsketch.com/$1 [R=301,L]