It's a bit of a paradox, I'm using the 32 bit version of Ubuntu on the PC while on one of my mobile phones a 64 bit based operating system is in operation.
There is one particular reason that made me still use the 32 bit version of Linux on my current notebook. When I set it up around two years ago I wasn't sure how backwards compatible the 64 bit version of the OS would be for 32 bit Windows programs I still have to use every now and at then for compatibility reasons via Wine. Perhaps it's no issue at all but I saw now risk or disadvantages of staying on 32 bits as Linux unlike Windows 7, which is restricted to 3GB on 32 bits, can still use the full 8 GB of RAM on my machine via PAE (Physical Address Extension). Each program is restricted to 4 GB of addressable memory but on a notebook not running enterprise scale applications that is not an issue at all. But it's definitely a kludge to ensure backwards compatibility.
As there are fewer reasons on mobile platforms to be backwards compatible it is quite a logical step to start the transition to 64 bits now. Some devices such as tablets already have 2GB of RAM inside today and going beyond the 4GB threshold is likely not far away anymore. So it's a good thing companies are thinking about 64 bits now instead of coming up with things like PAE on mobile just for the sake of dragging along old stuff.
So there we go, I'll have to live with my 32 bit PC versus 64 bit mobile paradox for just a little while longer.