I have an Android 1.5 project in Eclipse and whenever I merely have an xml resource file open the build dies with an error. E.g. if I have the res\\layout\\main.xml
You should be able to have the xml file open, and build. There is an option if you right click on the project and go to android to clean up the project or reset the properties. Try that and see if it resolves the issue.
I had same issue then trying to build project with ant. So I just cleared bin/ directory and the project was build.