I have the following test class that uses generics to overload a method. It works when compiled with javac and fails to compile in Eclipse Helios. My java version is 1.6.0_2
It would be possible if javac had a bug in it. Javac is just software and is as liable to bugs as any other piece of software.
Plus, the Java Language Spec is very complex and a little bit vague in places so it could also be down to a difference in interpretation between the Eclipse guys and the javac guys.
I would start by asking this on the Eclipse support channels. They're normally very good at picking up these things and explaining why they think they're right or else admitting that they're wrong.
For the record, I think Eclipse is right here, too.