I've been wondering recently, how long did it take after 32-bit processors came out for everyone to switch to 32-bit? 64-bit processors have been out for quite some time now, and pretty much any new computer you buy nowadays is 64-bit. Yet the support for 64-bit systems, while growing, is still fairly limited - unless you're in Ubuntu, in which case pretty much everything works, I'm told Flash is a big pain to set up in Gentoo for 64-bit versions of the OS.
In my efforts with FreeGamage, I discovered many games which wouldn't work right on 64-bit. I dig through the code a bit and a lot of the time it's because they assume pointer sizes are the same as int sizes, which on my machine they are not. Pointers are 64-bit, ints are 32-bit. This leads to some problems, as they'll assign a pointer value to an int value, which cuts off the higher 8 bytes and then there are segmentation faults when they attempt to convert the int back into a pointer and dereference it. This is bad practice, but it doesn't stop people from doing it.
I believe that the big switch to 64-bit computing probably won't come for a while. Why? What are the benefits of switching to 64-bit? It's hard to say, but I don't know if it is that much faster than 32-bit. You lose compatibility with many 32-bit programs. You need different drivers - even in Windows this is a problem.
The major limitation of 32-bit that I can see is when it comes to RAM. Let's do a little math. In theory, a 32-bit register (ie. the memory address register used to access RAM) can hold 232 possible combinations. 232 = 22 * 230 = 4 * 230. As most of us geeks know, 230 is the size of a gigabyte. So the maximum amount of RAM that can be accessed by a 32-bit register is 4GB. In practice, I'm guessing it is slightly smaller than this because Windows XP on my desktop can only see 3.2GB, when the system has 4GB. On a 64-bit system, you can theoretically have up to 16 * 260 = 16EB of RAM. We don't even see EB in practice yet, not even on a hard-drive. Hell, we don't even see PB in practice yet, which is 0.1% of an EB. EB is exabyte, which is a million TB, or a billion GB. That's a lot of RAM. Maybe we'll see EB hard-drives by 2020, but RAM is still pretty far away.
Because of the limitations on RAM, anywhere you see RAM intensive processes, you should see it switching over to 64-bit. On the desktop, I'm thinking one of the first ones to do this will be games. Games demand gobs and gobs of RAM, and coupled with Vista, which also demands gobs and gobs of RAM, you're probably going to be hitting your 32-bit peak pretty quickly. A nice 64-bit system will be able to handle all the RAM that games will require in the next few years or so.