I\'ve had some success with soaking in Simple Green. Of course, that strips paint, too, so if you\'re wanting to keep the paintjob, it\'s probably best to try the x-acto knife method.
I\'ve also had success with Castrol Super Clean. >>WARNING<< This stuff is very caustic, and will eat your hand. Make sure to wear rubber gloves when using it. A soak of an hour or two is generally enough.
I\'ve used the Simple Green on pieces that had greenstuff, and it didn\'t seem to harm it at all; it was a little soft immediately after removing, but recured within a couple of hours. I have not yet tried Super Clean on greenstuff, but I kinda think it will eat it.