As a follow up to my last post, I also tested out mapping subdomains with CPanel on a WPMu installation using Donncha’s Domain Mapping plugin. And surprise, surprise, just about the same method works for mapping just a subdomain to a blog on WPMu. To clarify, when I say mapping just a subdomain I mean mapping just one part of a domain, rather than the entire domain. For example, I don’t want the domain jimgroom.org to only host one WordPress blog because I plan on using this domain name for other things like a MediaWIki or even a Drupal installation 
So, all I do is create a subdomain such as blog.jimgroom.org and map that to a blog on my WPMu installation.
Assuming the domain is already pointed to your host and you are using CPanel like me, just create the subdomain and point the document root to your WPMu installation. In the following example I added a subdomain blog to jimgroom.org and then pointed it to my WPMu installation using the Document Root field (which is at public_html/wpmued-org for my personal installation).
After you do this, you can map subdomains for all your favorite domain names to one WPMu installation and save yourself the headache of updating numerous blogs on numerous domains that need numoerous updates and themes and plugins and whatnot. One installation to rule them all!
Zach