Well, since I've got 100Mbit/s I would have no problems?
Streaming is so much smoother, and easier, even though you can get better quality on a harddrive or a disc, with more space to store the movies, hell, 1080p is not the best you can get and blu-ray will get replaced given time. Streaming maybe only gives 2.1 or 5.1 sound, but it smooth, fast (no 30sec-2min before you can watch your blu-ray disc), it's cheaper than buying and it takes less space in your living room.
I'll probably buy a blu-ray later on, when they are cheap, have all their standards worked out (internet, hdmi 1,3 etc.) and they have a decent quality on all the movies (still too many crappy blu-ray releases). An addon to the 360 with blu-ray support would be to late, to expensive, and of course stupid (license money to sony). I think I'll go with streming from the net or getting a cheap media extender (like twixer or something) tyo watch all those movies in "full quality" XD
Everything better than dvd is av victory according to me
Anyways, Streaming HD Content isn't a good idea IMO, it takes too much bandwith and it pretty much has to be the only thing going on while watching that movie, meaning I can't watch a movie while my roomate plays Counter Strike or something. Plus not to mention I'll bet only 15-20% (If that) of the XBL users actually have the capability of streaming TRUE 1080P video, as others have said.
I honestly think blu ray wouldn't be that expensive to have as an addon for the 360. The only thing that they need to really sell is a Blu Ray drive, with a SATA to USB Converter internally, which you could build for ~$130. They would just have to take the time and incorporate the Xbox to read the format (Obviously). I personally think that if they added blu ray then there would be no specific reason for me to get a PS3, cause that's the only reason I would get one.
I understand that a standalone isn't that expensive nowdays, but I don't like having 10 devices around my television. I like having an all in one type thing, hence I use the Xbox 360's Media Center addition as an HTPC as opposed to having a dedicated HTPC. Sure it has it's pro's and cons, but I want as few devices as possible.