We're going to need much more information to help you solve it. Probably the contents of the zip.
Thanks Ignacio & Ollie.
For the sake of completeness, and in the (admittedly unlikely) event someone else may encounter a similar issue and find themselves on this thread, it turned out to be the silverlightmediaelement.xap file in the Assets folder that the LMS rejected. Deleting it fixed the issue. Happy days!
Cheers