adamwotherspoon on "How to add "www" in front of all permalinks"

Hi
I'm wondering if there is a way to always have the "www" in front of all urls/permalinks. Right now, every wordpress site I build deafults all url's to just "http://domainname.com" but i really need it to be "http://www.domainname.com". Especially for this one site I am building b/c my client uses the domain name w/out the "www" for an internal company site. so i need to make sure all the pages urls have the "www" in front. I've done some searching but haven't found anything. Any help would be appreciated, thanks!