Right now there aren't many programs that are designed specifically for 64-bit processors. In fact Windows XP Pro. 64-bit just came out a couple of months ago. 64-bit processors and OSs still run almost all 32-bit programs, aside from those that are directly integrated withyour system, such as antivirus software and drivers. Thus, most of today's programs will still be fully functional in the new system.
Then, since Microsoft will ship 32-bit versions of Windows Vista, I doubt there will be a major switch to 64-bit programs (which can only run on 64-bit systems). It just doesn't make sense to produce only 64-bit programs and neglect a large portion of the market. In addition, I don't think many programs will be offered in both 32-bit and 64-bit versions any time soon, as a 32-bit version will run fine on all systems...why would they spend more to produce 2 versions when one will get the job done and make everyone happy?
I'm guessing that only high-intensity games and graphics programs will switch over to 64-bit in the near future. The industry is hoping to have mostly 64-bit computers on the shelves by 2007, and stop offering 32-bit computers sometime after that. When 32-bit computers completely disappear from the shelves and the next version of Windows (after Longhorn/Vista) is released, software designers will focus on 64-bit programs and many will leave 32-bit users in the dark. However, that is years (probably 3-5) away. Until then, the average user will be fine using 32-bit systems and shouldn't have a problem finding future versions of their software.
Hope this helps,
John