When connected via wifi, the stream comes through with no restrictions, i.e. resolution was as normal and speed over wifi was at a bitrate of 1200 - 1400kbits/s.
When connected via 3G, the resolution is downsized to the screen size of the iphone (or iPad if you had it) so in my case to 352x240 and a bit rate of 200kbit/s. Streaming was clear as daylight, picture didn't look any worse at all and sound was perfect. Not all files use the same bit rate as some have been 312, others 201 and some at 140, so would depend on how your PC decides to encode it, but it's all h264 codec so smaller than normal and very clear picture.
I calculated that at 1hrs worth of streaming movies off your PC at 200kbit (obviously there would be some overhead too) that a movie would be 87.89Mb of my data usage. Not too shabby that it streams so well over XT network :)
I guess when data rates go up as far as 10gb a month for mobile, this could be the future of mobile traffic provided the networks can keep up the good speeds. I'd be interested to know how well it streamed over vodafone if anyone here is keen to try :)