Ok, you've probibly heard about this before. However, the domain mapping and multi domains plugins cause a redirect loop when activated and single sign on options are chosen.
This is easy to reproduce, all you do is create the mutisite with subdomains, have a couple of mutidomains with subdomain sites, then try visiting them one by one when logged in.
If you clear browser cookies the problem goes away for a bit, until you create another subdomain site using a domain from mutidomains.
Please investigate and release new version of these plugins to resolve this issue.