I always love these questions and hope a little bump will remind someone with an answer to respond.
Darn if your first post wasn't on US tax day. That alone could be why it was missed. Hang tight and lets see what pops up.
RewriteCond %{REQUEST_URI} !^/users/sites/whatever
RewriteCond %{HTTP_HOST} ^whatever\.mysite\.com$ [NC]
RewriteRule ^(.*)$ /users/sites/whatever/$1 [L]
The first condition makes sure that the internal subrequest generated by the rewrite doesn't get processed by the same rule again (which would result in an infinite loop).
The second condition checks for the subdomain (case insensitive with the [NC] flag).
And finally, the fairly obious rule.