I think there might be a different driver in the HD 360 Pro than there was in the PX 360.
http://www.head-fi.org/t/615647/modding-px360 found this thread on modding them (Removing some isolation foam to boost bass)
comparing this: http://img833.imageshack.us/img833/4829/dsc03516x.jpg
to this: http://cdn.head-fi.org/e/e2/900x900px-LL-e209211e_IMG_0641.jpeg
It's something different, but that could also be the difference between left and right earcup. It could also just be the housing of the driver that's changed. It doesn't look bigger or smaller.
Oh, and the names for this casing? Glob globbit, there're many! PX 360, PX 360 BT, PXC 360, HD 360 Pro, MM-500-X and MM-550-X.
I'm not too sure about all the differences.
Wired: PX 360, HD 360 Pro.
Wireless: Everyone else, optionally corded (Does the mic/remote still work corded?)
SRS: PX 360 BT, PXC 360 BT, MM-550-X
ANC: MM-550-X (Anyone else?)
I emailed Sennheiser in a couple of ways about it, it was mostly a fanmail, though.
Slightly off-topic: Anybody here who used these, who know of a better/bigger/more fun sounding alternative with the same kind of fit? I've been out trying stuff for a month, trying to find a replacement for my broken PX 360s (The wire snapped somewhere after 2.5 years, since their inception), but nothing isolates just as well.