So, I was having the same issue, but trying to import Android code via the "Import..." menu. When neither of the above two solutions worked on Eclipse Juno:
Eclipse -> File -> Import -> General -> Existing Project Into Workspace (NOTE: NOT 'EXISTING ANDROID PROJECT')
(Projects should import correctly, but should have errors. We must now attach the SDK to the project)
Right-Click on the project, Properties->Android->Project Build Target Choose the appropriate build target (in doubt, use 4.0.3 in the project is newish, and use 2.2 if the project is oldish)
Click OK
Once the project rebuilds, everything should be back in order.
(This was written when Eclipse Indigo was in vogue, and there may be changes as Google updates their tools to cover corner cases.)