domain mapping and the missing www

hi,
Just a quick question probably already answered many times (try to search with www as a string ) sorry about that.

How does it come that http://www.whatever.com is redirected to whatever.com once mapped using your plugin ?

Any way to get back the www before the domain name ?

Thanks

(edit : I just saw another thread talking exactly about this issue, next time I will search better. My question still stands though, especially in the light of "This plugin essentially allows you to allow all of your users to map one (or multiple!) domains to their own blogs". )