In a post at ProVideoCoalition, Simon Hayhurst, Adobe's Sr. Director of Product Management for Dynamic Media, confirmed that After Effects & Premiere will be 64-bit only in CS5. See The Future is 64-bit at PVC [note: Autodesk discreet smoke is 64-bit already] and the FAQ at the CS4 Production Premium page. From the FAQ:
"Adobe continues to prioritize 64-bit support based on the potential user benefits and the complexity of the code transition. At this time, After Effects and Adobe Premiere Pro are the only products that we are announcing will be 64-bit only in the next major release."
There's a bit more on this from AE Product Manager Michael Coleman in The future of After Effects is 64-bit native. At this point 64-bit might be a problem only for some people on Windows, but there's plenty of time to upgrade before CS5 (next Spring, assuming the usual release cycle).
Well, that's not quite right, since all your old plug-ins -- on Mac and Windows -- have to be 64-bit too, unless there's some emulation. As Frank Wylie remarked on the AE-List , "Looks like some late nights for the after market plug in folks!"
Update: Dav's Techtable there's more on PremierePro in It's Official: The future of Adobe video is 64 bit