Hi Dave,
mojoPortal doesn't work exactly like DNN, but it may suit your needs. I don't know much about the DNN sub portal feature.
mojoPortal currently allows hosting multiple sites under a single installation and db as long as each site has a different host name.
In other words you can have
http://host1.somedomain.com
http://host2.somedoamin.com
http://host1.someotherdomain.com
all running from one installation and 1 db, however each site is completely independent and has different users
There is documentation here:
http://www.mojoportal.com/muliplesites.aspx
I also have a customer who is sponsoring development of a second way to have multiple sites using a scheme like:
http://host1.somedomain.com/site1
http://host1.somedomain.com/site2
This feature will get implemented sometime between now and September. Again each site will have different users and be completely independent.
Hope it helps,
Joe