Yes, "Hide FileMaker Pro 10 Advanced" worked for me on my iMac running OS X 10.5.6.
This is all handled at the system level, so my gut feeling is that there is a conflict somewhere in memory. I would try and reboot the entire machine. Launch FileMaker Pro and Address Book. See if you can now Hide FileMaker Pro, and conversely, go into Address Book and see if you can hide Address Book.
Over the weekend, I belated upgraded from 10.5.5 to 10.5.6.
All is now well with Hide FileMaker on 10.5.6.