Just get one of those AV senders. Fairly cheap and wireless. I had used some back in the UK before getting Sky Multiroom. Dixons and Argos used to sell them. Only downside is you have to watch what the main box is watching, but you can get some with remote control extenders that allow you to change channels from another room. Picture quality is normally pretty good. Oh, and some things can interfere with the signal. I remember it used to stop my wireless G laptops from working (wireless B worked fine), but after mucking around with the settings (you can select different frequencies) and models I found a setup that worked.
With MiVision though I doubt there is a true "multiroom". Basically the ability to watch one thing in one room, and another channel in another room. Mainly because it uses the internet and you are are allocated a certain amount of bandwidth (about 3 mbits I believe). They would have to give you more bandwidth to have a true second MiVision box, and you know how damn scroogy Cyta is. Anyone know when they are going to be increasing the speeds BTW? Was supposed to be start of september, but seems to have been forgotten....
Cheers