I just spent 40 minutes trying to work out why Tiger (10.4.6) was refusing to mount disk images, Google didn't have the answer, so hopefully after publishing this, it will :-)
Double clicking a .dmg file gave no response, and the Disk Utility, hdiutil and hdid commands locked up. Plugging around with the -debug and -verbose options gave nothing away. Resorting to trial and error clearing cache files from under the /System/Library/Caches directory was the answer with deleting Extensions.kextcache and Extensions.mkext then rebooting finally doing the trick.