Of course with many discs these days a digital copy is included. I would love to be able to designate within Plex Media Server when I have a copy of a title available via one of those services. I will sometimes make a reference in the notes section for a title. What would be really nice is to be able to launch that Vudu or Amazon copy (or iTunes once Plex is available on Apple TV) directly from within Plex such that the Vudu app is then opened on a Roku and directly on that title.
I guess a corresponding feature request would be to be able to add movies to your library that you don't actually have on a local hard drive. Then Plex could serve as an aggregator of all your "owned" media, whether or not you have it on a local hard drive or it resides within Vudu, etc.
This would also be useful if perhaps you own a DVD quality version on a local hard drive but perhaps purchased the HD version from Vudu, rather than purchasing the blu-ray. You could keep the DVD quality version on a hard drive but be able to browse and launch the HD version onto Vudu. Could also function as a space saving measure (keep an SD quality on the hard drive, but have the option to watch the HD version via Vudu).
Anyway, just some thoughts.