I feel a bit silly that I can't think of a reliable way to get this information.
SPUsers, but not all, now have claims
LoginNames. This makes parsing the domain name a little bit tricky. Is there a clear method somewhere that doesn't care whether they are claims or regular domain\username?
I have found the
SPClaimProviderManager type, which does help, but I would have thought there would be an easier way to extract this information.