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.
No
java.lang.Object cannot be resolved in Eclipse
This problems started to appear recently for me after I upgraded ADT. As suggested above, when I uncheck+Apply/check+Apply project build target it would help , but the problem would come back eventually. So instead I just added android.jar explicitly to the build. This is the entry in the .classpath file
This is more of a workaround as I would have to change it every time I change the SDK version, but at least no need to play with properties to get it to build when I restart Eclipse.