I just loaded an FP12 runtime on an iBook running Tiger, and it did not work.
If you look into the runtime, you see it has some left overs of PPC code, but the main app has no PPC code.
FileMaker Pro 12 and FileMaker pro Advanced are Intel only. No support for PPC.
The basic engine beneath FileMaker Runtime is the same as for FileMaker Pro.
I just tried FMP 11 on a Tiger machine and that didn't work either.