Use mod_rewrite to create "fake sites" (with subdomains) on a single apache vhost

Note: the subdomains need to be server aliases on the vhost

RewriteCond %{HTTP_HOST} ^(subdomain1|subdomain2)\.yoursite\.com [NC]
RewriteRule ^(.*) http://www.yoursite.com/%1/%{REQUEST_URI} [L,P]

Apparantly this also works:

RewriteCond %{HTTP_HOST} ^(sub1|sub2)\.yoursite\.com [NC]
RewriteRule (.*) http://www.yoursite.com/%1/$1 [R=301,L]