My house has 3 desktops, 1 server, 1 netbook, and 1 smartphone (I don't count my girlfriend's Blackberry as a smartphone). I'm sure I'm an unusual case, though.
Yeah, the desktop market is pretty saturated while the mobile market is growing quickly, so everybody is making a mad cash grab at the mobile market right now. I'm sure it will level off a bit, but it's also a lot easier to justify building planned-obsolescence into mobile devices, so I'll bet there will always be more device turnover there.
Also, loads of people buy a desktop and keep it for a few years (or a decade, even), but upgrade their smartphone every year or two. So not only is the market growing, it's also got a higher turnover of devices.
'Course, this is all anecdotal, so the real figuresmay be different. But given the actions of various companies, I'm probably right.
EDIT: The higher frequency of upgrades would be because telephones have become so central to so many lives, while microcomputers have kinda stayed in the background for a large chunk of the population. Plus, your much more likely to see the fancy new thing the Jones' have if they carry it almost everywhere.