r1000 need some change,not only recompiling.
And you are now fully right.
10.6 sdk support kernel extensions x86_64 and i386.
10.5 support only i386.
the people usually say 32 and 64bit,it's true to windows system for example.on windows64 all component have only 64 so 32 run in emulator.
10.6 can ran 32 application(not kext) in 64 mode and no emualtor used.
original 10.6 system application and kext have hybrid x86_64 and i386 so they run in 64 and 32 mode but in 10.6 not 10.5(kernel doesn't understand 64 even hybrid i386&x86_64).
but some apple kext has only i386 in 10.6. I don't know why.
It's possible to slim kext and app in x86_64 arch only then they're 64kext,else it's hybrid.

most old kext from 10.5 still work in 10.6 but they're old.
nalivay poidet.