That would be nice especially in JRemote,
although would have to be completely optional especially for people who connect through 3g. Don't know what extra overheads there would be, but I'd imagine that the thumbnail files would be significantly larger unless the preview images were lesser quality than the static image that would reside when a video is not selected or the mouse is hovering over.
come to think of it I would really like this feature, for me it would breath some life into mc making it seem more interactive, as it would help somewhat in the selection of a video to watch.
It would have to be subtle though, it wouldn't be nice if the mouse was dragged across the screen and several previews were activated - only one at a time, but it couldn't be too subtle that it feels like it takes too long for the preview to kick in