Not to be Donald the Dork here, but not all 1080p streams are made the same. Blu-ray offers a higher bitrate (meaning more quality, less compression) than nearly all downloadable 1080p sources. Too tired to find the linkage, but if the finished file is smaller than 25GB (capacity of single-layer blu-ray disc), then the Blu is likely better.
This is why I have no real desire to buy movies from iTunes. (in any case, iTunes only offers 720p currently)
Yes, iTunes uses h.264 which is one of the best video codecs currently available... However, it's just part of the MPEG-4 spec which is also used for Blu-ray movies.
Thus a Blu-ray movie is likely using a similar codec (if not the same h.264 codec) for video. I don't foresee iTunes having 25 GB movie files for download in the near future. (especially since that'd take up a lot of space on most harddrives)
Even if iTunes offered 1080p movies for download, it's very likely that they'd be compressed down to 5 GB (maybe more, but probably not). That's a fifth of the space of a SINGLE LAYER Blu-ray disc. I think some movies might even use two layers. Sure, as the file size increases, the loss of quality becomes less and less noticeable, but I think most people would be able to tell the difference between a 5GB movie and 25GB movie.
Another thing Blu-ray offers that Apple probably won't for quite some time: Lossless audio. (sometimes uncompressed, which I don't really understand... there's plenty of great lossless codecs out there, but I guess they don't need to worry about space on Blu-ray discs) So iTunes movies (if they even had surround sound) would still have compressed audio, which would also be another downside.
Apple really just needs to get off their "We know best" mentality and put Blu-ray into their devices. Not doing so isn't going to make me want to buy iTunes movies... it's just making me hold off on getting a new Mac (like Mac mini --HDMI would also be nice-- or 27" iMac).