Multisite, Domain Mapping, and Subblogs

Update: You should be able to achieve this now using our new Multi Domain plugin - enjoy!

I am trying to figure out if it is possible to do a setup on a single WPMU install like this:

I've been trying to figure out if this is possible by playing around with a 3.0 install with multisite enabled, the WPMU-DEV Domain Mapping plugin, and manually writing some mod_rewrite rules in the .htaccess at the root.

Is this at all possible? Thanks.