For 7 months since a release of Snow Leopard in August 2009, not all bundled applications are 64-bit capability. One of that is a flagship application, iTunes. iTunes is an all-in-one application for iDevice syncing (iPod, iPhone, and iPad), media management, and media purchasing via iTunes Store.
So what is wrong with that? The problem is iTunes user interface that still uses Carbon. Because Apple has deprecated 64-bit Carbon user interface in Leopard. iTunes depends on a lot of Carbon user interface elements, ie. Apps list in iPhone Apps tab. I don't know design decision, why not moving to Cocoa. One of possible reasons is for platform compatibility because iTunes runs on Tiger, Leopard, and Snow Leopard. Cocoa in Tiger is outdated comparing with Leopard and Snow Leopard. Creating user interface elements with an outdated Cocoa may be difficult when comparing with Carbon. It may be too complex even Apple doesn't want to refactor it!
I hope a future version of iTunes will be a 64-bit application. I don't know why should it be but iTunes uses more and more resources. It is too big when comparing with the original iTunes.