I use the following code in .htaccess:
RewriteCond %{HTTP_HOST} . 
RewriteCond %{HTTP_HOST} !^davidbowley\.com
RewriteRule (.*) http://davidbowley.com/$1 [R=301,L]
...to make all http://www.davidbowley.com go to http://davidbowley.com and it worked perfectly.

Except now I've been adding addon domains to my account in cpanel, the code completely screws with them. .htaccess affects all the subfolders, including the ones for addon domains, and it forces www.mysite.com to redirect to davidbowley.com/mysite constantly.

I've removed the code for now but it would be great if someone could show me a work around. I don't understand RegEx, I'm more of a copy/paste kinda guy.