Use the lipo command line utility, which strips architectures off fat binaries (what an appropriate name). First, check which architectures there are in your Growl framework:
$ lipo -info path/to/Growl.framework/Growl
Architectures in the fat file: Growl are: x86_64 i386 ppc
In this case, we simply have ppc, but there are about 10 variants (of which I've met 3). To avoid any surprise, you should run this command any time you want to strip architectures from a file instead of just jumping to the removal part. (If you're curious, man 3 arch has the exhaustive list of possible architectures for fat binaries on Mac OS.)
Then, remove the ppc achitecture:
$ lipo -remove ppc path/to/Growl.framework/Growl -output GrowlIntel
Find the real Growl binary (should be under Versions somewhere) and replace it with GrowlIntel.