I recently updated to Ubuntu 12.10 and Eclipse 3.8 Indigo. My ADT plugin was gone, and I was having trouble installing it again.
I was getting this error in the software installer:
Cannot complete the install because one or more required items could not be found. Software being
installed: Android Development Tools 12.0.0.v201106281929-138431
(com.android.ide.eclipse.adt.feature.group 12.0.0.v201106281929-138431)
Missing requirement: Android Development Tools 12.0.0.v201106281929-138431
(com.android.ide.eclipse.adt.feature.group 12.0.0.v201106281929-138431)
requires 'org.eclipse.gef 0.0.0' but it could not be found
The solution was to rm -rf ~/.eclipse/, then run eclipse as root and run the update check, and update everything.
Seems to have fixed it - I was able to install the ADT plugin. You might be prompted to update some SDK packages when you restart but it's fairly straightforward.