Quick! Come up with three good reasons to switch to a 64-bit desktop OS. I bet that overcoming the 4GB memory limit (or, more accurately, the 4GB addressable space limit) is one of the reasons you stated (if not, I'd like to hear what three reasons you came up with).
Windows Vista has changed the ballpark that OEMs have to work to. Not only does the new OS like a decent graphics card (if you want to run Aero at any rate) but it also needs considerably more RAM than XP. In fact, even though Microsoft says that a Vista Capable PC only needs 512MB of RAM, personally I think that anyone who who tries to work with Vista on a machine with 512MB of RAM is pushing their luck. Spend your money on the hardware before the software.
Personal note: I really did think Vista would mean the end to OEMs being able to sell cheap, underpowered, mediocre PCs (CHUMPs) but it hasn't, although they are easy to spot because these are the systems running Vista Home Basic.
However, now that 1GB is at the bottom end, that means that 2GB of RAM (an amount that would have seemed lavish not long ago) is middle of the road territory. This means that the 4GB limit imposed by 32-bit operating systems isn't all that far away. In fact, memory is so cheap that not loading a PC up with 4GB seems crazy, especially if you're willing to spend money on Vista. After all, you can load up on 4GB of DD2 677 for $150.
But ... if you're running a 32-bit OS you're not going to see 4GB even if you do spend the money. After everything else has taken its share you don't end up with 4GB, you end up with only 3GB. The OS swindles you out of 1GB. The fix? Jump to 64-bit.
And this is how I see the migration to 64-bit OS happening. Sure, it'll be a slow migration, but as far as I can see breaking the 4GB RAM barrier is the main reason to make the switch. It'll probably start with the gamers and those who like to dabble with photo and video editing because these are the folks who'll need the extra power. Slowly others will follow. And remember, while 4GB of RAM will set you back $150 now, in a year you can expect that price to drop substantially.
Microsoft hasn't made any decisions as to whether the next version of Windows will be 64-bit only, but personally I doubt that Windows 7 will be, especially if Microsoft expect to ship it in a couple of years. I just don't see the market as being ready for the shift. But by the time Windows 8 is ready to make an appearance, we might be ready to abandon 32-bit computing.