Just following up on this one, as we are having a similar problem with a GWT 2.0.4 project which we have just inherited, and which has all of the java/uibinder stuff in src/main/java, but all of the referenced css files in src/main/otherfolder (same project, but different source folder). Everything builds fine from the command line, but none of the css resources can be resolved in Eclipse. If I move one of the css files into src/main/java, then the css is resolved by Eclipse. So I suspect that this is the GPE failing to resolve resources on the classpath in source folders other than src/main/java. Yes? No?
FYI, we are using GPE 1.4.2.v201012211742, which was released 3 months after Eric's post, so I'm a bit puzzled as to why we would still be seeing this behaviour.