Welcome to MobileRead, AMacD -- you left out Vista from your list.
Yeah, there have been some rumors that they're looking at Macintosh, but the limitation on the OS's is a bit puzzling. The only thing I can think is that from the way the Connect Software is so marginal in several areas, maybe there just weren't enough resources allocated to it. Just a guess of course.
One thing to note, I'm pretty sure that the PDF reader app on the Reader is supplied by Adobe. So one could be excused for thinking that it works as well as Adobe
wants it to work.