I got to update to the latest version of the ADT tonight when I hit the weirdest error out of the blue:
Unable to build: the file dex.jar was not loaded from the SDK folder
After way too long of Googling and not coming up with an answer I finally found it on some really obscure mailing list archive site.
Look in the project with the error and delete the default.properties file. That is it.
Just make sure you have the latest versions of Eclipse and ADT when you do it, and that should be it. Back to Android programming...