Ron Rennick on "[Plugin: WordPress MU Domain Mapping] CNAME and DM example"

Is there any way to retain mydomain.com in the URL bar instead of showing the redirected site1.otherdomain.com?

Site ID = 2 (site1.otherdomain.com)
Domain = mydomain.com
Primary = NO

If you want it to stay in the address bar make it the primary URL.