Pd itself has been 64-bit ready since version 0.41.4, and Pd-extended is now fully 64-bit on GNU/Linux with 0.42.5. Mac OS X 10.6 Snow Leopard is fully 64-bit now, so its a good time to start making fully 64-bit builds.
One big problem with making a full 64-bit build of Pd-extended for Mac OS X is that they have deprecated some of the Carbon APIs and will only support them in 32-bit mode. Gem, hid, and pidip all rely on those APIs, so they won't work in 64-bit without some major surgery.
Here's how to get a 64-bit build of Pd-extended to finish:
install a build of Tcl/Tk 8.4 into /Library/Frameworks
in packages/Makefile remove gem from the line beginning with all: and gem_install from the line beginning with externals_install
copy libPdTcl.dylib from a 32-bit build of Pd-extended