Jeff,
I want to point out a related issue:
You can't login into separate subdomains (i.e. they are
actually subsites, but are mapped to subsite nodes in the host-
node map), instead you have to login to the main site.
There are few threads on this already:
A lot of people would like to see this resolved, but its a pretty
hairy issue and nobody has found the time yet. I'm not expecting
you to take this up, but if you find anything that might be helpful
on this front while you are digging into subsite registration it
would be great if you could report back.
TIA,