Domain Mapping: www vs non-www

I'm new on this forum - learning a lot from you guys - thanks a lot! - but can't seem to solve this....

I am using WPMU to host a multi-country website. Each country will have its own website - e.g. http://www.domain.com, http://www.domain.co.uk, http://www.domain.ca etc...

I've installed the WPMU domain mapping plugin but I can't get the site to resolve to the "www" subdomain for each site which I need. I tried adding an entry in the htaccess file but this just causes a loop and times out without resolving.
Is there any way round this? Thanks.