I upgraded my master to 5.0_MP3 and had SSO media servers running at NB4.5_FP3, NB4.5_FP6, NB5.0, and also NB5.0_MP3. There were and still are no problems with SSO.
I have also found no warnings in the release notes or install guide about 5.1 upgrade problems in a mixed environment.
One thing to consider is that the SAN server license is enforced in 5.x and you cannot do client backup/restores to any node other than the SAN server itself.