“Unable to get system library for project” after I upgraded to Android SDK 2.3 and ADT 8.0

Today I upgraded to Android SDK 2.3 and I also upgraded the Eclipse Plugin Androi Developer Tools 8.0 (from 0.9.9). I also upgraded my Java Development Kit to 1.6_22.

Now in Eclipse I get many errors in all my Android projects.

The error messages are like:

The type java.lang.Object cannot be resolved. It is indirectly referenced from required .class files.

How can I fix this so I can use my Android projects again?

In the Build Path for these projects, I don't have any references to an android.jar but a message:

Unable to get system library for project