I've learned that error messages like this are usually right. When it couldn't POSSIBLY (in your mind) be what the error being reported says, you go hunting for a problem in another area...only to find out hours later that the original error message was indeed right.
Since you're using Eclipse, I think Thilo has it right The most likely reason you are getting this message is because one of your projects is compiling 1.6 classes. It doesn't matter if you only have a 1.5 JRE on the system, because Eclipse has its own compiler (not javac), and only needs a 1.5 JRE to compile 1.6 classes. It may be weird, and a setting needs to be unchecked to allow this, but I just managed to do it.