I've just posted a document I'm working on, proposing a solution to storing thumbnail references, as well as various other formats of the same learning object, within a single LOM metadata record/document.
The Coles' Notes version is this: We need a place to store locations for thumbnail images for a learning object, as well as various formats (QuickTime, Real, Windows Media, etc...) without having to duplicate the LOM. The current solution would be to create separate LOMs for each format, or create a content package. Neither solution really addresses the problem.
If you're interested in the problem, please take a look, and let me know what you think. If you have some constructive feedback, I'd love to incorporate it into the document (giving attribution of course) and provide it to IMS so we can all benefit.