I guess I have been very lucky. I have a mac mini and I am running parallels 3.0 (I have ugraded from version to version.) I have been running Media Center under XP2 on Parallels for over a year. It has worked great.
I never used theater mode until last week. It worked only about 30% of the time. But, I upgraded Media Center to the latest build (I had been on a very old build) yesterday. So far the theater mode has worked 100% of the time and looks great.
All of my music (thousands of .ape files), photos, and library are on terrastation network drives. I have my Windows XP automatically mount them as a drive when it boots. This allows media center to start and see the files without a problem. This did not require any extra software (ok, parallels) , and allows me to run media center (as well as see the files) from my other computers as well.
I think that parallels on mac os is a great solution. I think that if a port is prohibitively expensive, then maybe the next best thing is for jriver to test media center under parallels. A mac mini and parallels is pretty cheap compared to a full port.