Main site URL without www sends users to registration page

This is annoying!

I have a multisite install with BuddyPress 1.5 running.

Visiting http://www.site.com works fine and you go to the main site's home page.

But, visiting just site.com (no www) redirects the user to http://www.site.com/register/

I'd rather this didn't happen.

Any ideas?