Ultraviolet's morass of logins and passwords hasn't gotten any less complicated for digital copies from different studios yet, but in this case linking your account should be relatively simple.
While the overall Ultraviolet sign-up process and its multiple logins hasn't gotten any easier, once we created a ParamountMovies.com account we were able to stream a copy of Hugo unlocked by the Blu-ray 3D version in HD just by signing into the free app (XBL gold subscription required) on the Xbox 360 -- no antiquated discs necessary.
Movie studios have tried to get Apple to participate in the UltraViolet effort, according to people familiar with the matter, but so far haven't succeeded.