The news is slowly leaking out, but the rumors have finally been confirmed by Adobe and released in this white paper.
Future versions of Premiere and After Effects (CS5 and up) will be 64-bit only, requiring Vista 64-bit or Windows 7 64-bit, and Leopard /Snow Leopard for the Mac.
With a 64 bit version of Premiere and After Effects, your applications will no longer be handicapped, and will once again be able to take full advantage of your system’s capabilities.
Read more about this announcement here:
Very exciting, as this is LONG overdue. We will of course continue to make sure our products are supported in all future versions of Adobe software.