Themes placed in sub folders do not work in preview mode.
|Reported by:||grandslambert||Owned by:||anonymous|
Refer to the discussion here:
A simple fix was suggested in that discussion, simply add the '/' character to the regular expression for themes and styles on lines 852 and 861 in wp-includes/theme.php so themes in subfolders can work.
I realize this may be more specific to WordPress MU, but similar questions have been raised for WordPress as well.