I created an account just to post here because this issue was driving me crazy as well. I finally tracked it down (at least my problem).
I had installed some guitar effects processor demos for Amplitube and Guitar Rig. After I uninstalled the demos, that is when I started getting the registry errors. I removed all Garageband related files on the whole drive (sudo find / -iname "garageband") and re-installed from my DVD and still got the registry errors. I registered both my Macbook and iLife '11 with Apple like someone suggested, no dice.
Finally I searched the drive for leftover Amplitube and Guitar Rig files and found stuff leftover all over the place but specifically in both /Library/Audio/Plug-Ins/Components and /Library/Audio/Plug-Ins/VST:
10:17:53 denisesballs@denisesballs-mac:/Library/Application Support$ cd /Library/Audio/Plug-Ins/Components/
AmpliTube 3.component/ Guitar Rig 5.component/
10:19:01 denisesballs@denisesballs-mac:/Library/Audio/Plug-Ins/VST$ ls
AmpliTube 3.vst Guitar Rig 5.vst
I removed those 4 folders and voila! I also removed the other Amplitube and Guitar Rig files from my system, but those appeared to be the ones that fixed the error. Garagband now starts normally without any registry errors. Hope this helps someone.