I watched Eddie & the Cruisers the other night and had forgotten what a good movie it is. If you've not seen it, please do so ASAP.
I noticed something very interesting, though. About 7 minutes into the movie, after Frank asks his class a question about Wordsworth's "...Tintern Abbey," a student answers who looks and sounds exactly like Data from Star Trek: The Next Generation. The role is uncredited, though, so I haven't been able to verify if that's who it is or not.
(And while we're on the subject of unknown cameos, is that Robin Williams in the old west outhouse in Bill & Ted's Excellent Adventure? I've wondered about that for over ten years!)