I've installed domain mapping many a times now and every time, with or without plugins, it fails to work the way i believe it should.
The domains map perfectly, however, admin bar never shows when viewing the front-end of a subsite. It's as if i'm not even logged in to that site.
Just thought i would report this issue. I know many others have had the same issue.
In contrast, the free mu domain mapping plugin does work and does show the admin bar.
I wished yours did work though.