Stefan Minning posted A plug-in developer's thoughts on After Effects CS5 being 64-bit ...
While Stefan isn't a major developer he fills niches and is raising a question on many minds. The side effects on AE filters of Adobe's move to 64-bit is worrisome to users (see the Toolfarm survey results) since AE filter development across platforms and OS updates is often asymmetric.
One example of problems (elsewhere) has been with QuickTime on Windows. Since there's no 64-bit version of QuickTime, import/export of QT files haven't been available in the 64-bit Windows versions of Nuke, Eyeon Fusion, or Syntheyes. On the Mac, Apple does provide technology that passes 64-bit Quicktime requests to a 32-bit server process, but it's slow and codec-limited. There's likely little incentive for Apple to develop for authoring on other platforms and maybe even other applications. See Philip Hodgetts Why is QuickTime X like OS X? for a more optimistic view, and John Siracusa for more technical explanations of the delay in the Quicktime sections (page 6 and page 16) of his Mac OS X 10.6 Snow Leopard: the Ars Technica review.
While Adobe does have a good history of development and support to leverage, for example with QT and other formats in Premiere timelines, we'll have to wait to see how they adapt to the problems of QT in 64-bit Windows -- and if they end up providing some slow, short-lived, and expensive-to-develop emulation for 32-bit AE filters.
Update: see Stefan's comments below on simple software bridges to inexpensive 64-bit audio apps. Another example is jBridge.