MacInTouch Home Page: "My sister just got a new iBook G4 and asked me to configure it. So the first thing I did after going though the setup stuff was to update it 10.3.1. I then tried to mount a .dmg file but it failed. Checking the boards I found this to be a known issue with no fix from Apple. One of the threads put me on the path to the solution, at least for me.
What I found from the threads is that there's a plist file that gets created the first time you mount a .dmg file. This file is called com.apple.frameworks.diskimages.diskimagesuiagent.plist. When I search for the file it was missing. That got me thinking, could this be the problem. The .dmg file won't mount because the plist is missing.
So to test my theory I reinstalled 10.3, but this time before upgrading to 10.3.1, I first mounted a .dmg file. Then I checked to make sure the file com.apple.frameworks.diskimages.diskimagesuiagent.plist was created, which it was. Then I updated to 10.3.1, and now .dmg files mount just fine. So before upgrading to 10.3.1 make sure you have mounted at least one .dmg file so the plist is created.
It looks like I found a bug in 10.3.1 plists; if they are missing they are supposed to be recreated, like 10.3 did, but 10.3.1 failed to do so."