I gotta go with CobraAI on this one. For a typical commercial desktop application, where devs would normally struggle with the issues of 32 vs. 64-bit, it's generally a non-issue. Think of QuickBooks, Goldmine, and other such event-driven applications here. These apps don't do a whole lot that's terribly CPU-intensive.
And the tools abstract the difficult tasks these days. Visual Basic, Delphi, C++ Builder, C#, etc. The devs usually don't need to worry about the low-level stuff. It's a rare situation that even an application written in Java will have CPU problems if it is just a desktop app. Generally slowness is going to stem from a design problem or disk I/O.
For those apps, there's no reason to worry about faster and faster processors, because on a multi-core system, they will get a full processor's 100% attention most of the time. And with so many libraries already out there for making apps multi-threaded, even these simplest of applications will be able to benefit.
For more low-level applications where CPU really is a problem, writing multi-threaded code has been a good idea (when applicable, of course) for a terribly long time. Seti@home uses multi-core systems very effectively, and has for at least a year or two. Not all apps will be able to make much use of a 256-core system, I'll wager, but many will take advantage of the dual and quad core systems we have today.
And the tools, man! Check out things like IBM's Thread Building Blocks
for C++. Makes it dramatically easier to build a multi-threaded application than most old-school devs are used to. And because threaded apps don't need a single core per thread, as long as you can expect a dual-core system, your app will almost certainly perform a great deal better if you use threads everywhere it makes sense.
So when we get past the speed barrier once again, why would we stop with multi-core systems? Even for apps that don't need multiple cores, they allow Windows (or OS X or Linux or BSD or ...) to run more effectively just by being able to dedicate
one core to the app in question.
Unless it somehow becomes significantly cheaper to have a single-core system, I just don't see any reason they'd go away. And unless the development tools turn out to be crap (and from my (admittedly limited) experience of TBB, I just don't think that'll happen), there's no reason for devs to avoid multi-threading their apps when the average household computer is at 2 cores today or will be soon.