Nope, there never was a 5.5, that was just a snide coment from me.
There is an IRIX, Linux, Mac and Windows release of Maya 6. It looks as though it's still just Maya Complete for Mac, too.
I can't see any reason why OSX and the hardware it runs on being good enough to run all the bells and whistles from the Unlimited variety.
As far as hardware rendering, that wasn't available in verson 4, 4.5 or 5 either. That's one thing that left me confised as well. It seems as though the sgi hardware was meant for this sort of thing, so why not exploit it?
For new things in 6, there are a some controls in Mental Ray that allow for better use of HDRI and they finally unhid a few options that control settings in your Final Gather. I also noticed that they niow have thier own Hair system built in. No idea how it stacks up to Shave, but it still better than just a plain shader or Paint Effects that are difficult to animate.
One thing I did find frustating is that the bonus tools for 6 doesn't have any of the shaders that are available in the bonus tools for 5. So if you have anything that you used the default shaders and want to import, it could be incomplete.