Multiple Domains for users (always on)

Hello.

I seem to recall reading somewhere, sometime ago that it was possible to map multiple domains to a WPMU site so that each user would have their blog available at multiple addresses like this:

userblog.domainname.tld = userblog.otherdomain.tld = userblog.yetanother.tld

I'd like the above in addition to mapping domains to individual blogs like:

userblog.domainname.tld = userblog.otherdomain.tld = userblog.yetanother.tld = userblog.com

Please let me know if and how the above can be accomplished.

Many thanks
Ralf