Read this: http://weblogtoolscollection.com/archives/2010/06/02/wordpre...

Then read this: http://ottopress.com/2010/wordpress-3-0-multisite-domain-map...

I use this exact method (and a subdirectory, not a subdomain install) to run multiple domains off on a WP 3.0 installation on cheap, shared, GoDaddy hosting. No apache config editing whatsoever.