[Update: Apple released iTunes 8, and whatever it is, it is not pure Cocoa]
[Update: Apple did release a version of iTunes for 64 Bit Vista. I don't know how it matches up with the below speculation. ]
This is entirely speculation on my part. If I had insider information, I would not betray it. This is just my uninformed opinion.
Fact 1: iTunes as we know it is a Carbon application on the Mac. It after all is the direct descendent of SoundJam, a short-lived MP3 player for Classic Mac OS. Open up its package in the Finder and see things like a .rsrc file.
Fact 2: iTunes makes heavy use of QuickTime and the Mac Toolbox emulation layer which QuickTime for Windows contains. This is what allowed the same codebase to be deployed on both OS X and Windows relatively quickly. (OK, this may not be a fact, not having access to the iTunes code, but it seems darn likely.)
Fact 3: There is no 64-bit C API to QuickTime on the Mac. There is only a 64-bit Cocoa interface.
Fact 4: Pretty much all of that Mac Toolbox being emulated on Windows is no longer available under 64-bit compiles on the Mac.
Fact 5: All new Macs are 64-bit. Apple is in a good position to start promoting themselves as The 64-bit Company, as their path to 64 purity seems easier than Microsoft's, but it will take getting all their apps there first, and convincing their major developers to follow. I assume it was easy getting their Cocoa applications there, but any residual Carbon applications would be a nightmare.
Extrapolation 1: Even Apple will not be able to make iTunes a 64-bit application without a major rewrite. After you remove QuickTime, all the old Classic Mac APIs which are not in the 64-bit frameworks, etc., there is little left. Maybe much of the task specific onscreen drawing on the Mac is now done with Quartz calls, and the networking calls are probably being done with CFNetwork calls on the Mac, and maybe they are using WebKit to do the store, and that can all be salvaged, and hopefully the code is factored such that system calls are not sprinkled all over the place, but they are going to have to bite the bullet and just write a from scratch Cocoa application and call it iTunes 8.
Extrapolation 2: If Apple is not going to extend the life of Classic Mac APIs and QuickTime C APIs for 64-bit Mac applications, what are the odds of them doing so for the emulation layer on 64-bit Vista. Pretty low. There is no obvious quick way to get iTunes as we know it on Windows to run as a 64-bit process. I guess they could if they really wanted to, but it seems a pretty hacky solution and would basically obligate them to maintain an entire OS's SDK just to run iTunes and QuickTime.
Extrapolation 3: I assume Apple cares about the future of QuickTime and iTunes on 64-bit Windows. This is not a big market now, but will only get bigger, and Windows user are not going to accept running multimedia software in 32-bit emulation mode in perpetuity. They can do two things. They can create a new framework which can play and edit QuickTime content, perhaps a set of .NET classes, and write a separate version of iTunes which makes use of these classes. Or, they can bring Cocoa to 64-bit Windows, and run the same source based iTunes as the Mac. I suspect Apple doesn't want to do either, but iPod sales are important to them. They wouldn't want those early adopter 64-bit users running to Zune.
A little Googling on "64-bit iTunes" came up with this revealing alert text in the current Windows iTunes "This iPod cannot be used because the required software is not installed. Run the itens installer and remove itunes, then install the 64-bit version of iTunes." That makes me wonder if all this is going to fall out sooner rather than later. Or maybe it's an aberration.
Anyway, that's my uninformed opinion, and we'll see if it has any relationship to reality.
[Update: See this article about how Adobe is not releasing a 64-bit version of Photoshop until they can do the major re-writing required to remove their Carbon code.]