Quoting Victor Trac victor.trac@gmail.com:
You should probably rename your /sites/site1.com to /sites/www.site1.com if you want to access it via www.site1.com. That should make it work for both http://site1.com and http://www.site1.com.
This a matter of choice; Drupal semantics should find sites/site1.com if the URL given is www.site1.com or site1.com. The www.site1.com is the 7th hit in the filter and site1.com is the 8th hit in the filter based on the comments of settings.php.
Earnie -- http://for-my-kids.com