"Jon Gruber actually discussed this yesterday here. Scroll down to ""Itunes 9""
What makes iTunes such an odd duck for Apple is that it’s not just a Mac app — there are far more iTunes users on Windows, and but there is no 64-bit Cocoa runtime for Windows."
"It's hard to transition every app to 64-bit. It takes a lot of resources, especially for such complex apps. In Ars Technicas Snow Leopard review you can read more about the compromises Apple made with Quicktime X (it heavily relies on the old Quicktime 7 for a lot of operations).
The thing is, Apple is spread thin on a lot of projects: Mac OS, iPhone OS, MobileMe, iWork, iLife + all the hardware projects. For the moment, a 32-bit iTunes just works (barely, unfortunately, for me). You'll probably see a major redesign of iTunes with the next Mac OS iteration, when the OS will really be completely 64-bit."
