Just tonight I upgraded my WordPress install to a multisite network and I have encountered this very same problem. If I define MULTISITE as false in wp-config, all of my themes appear, but as soon as it’s set to true they are gone.
Unfortunately, I took the advice in this thread and checked, double-checked and even recreated my authentication keys and salts, but to no avail. Does anyone else have a suggestion as to what could be causing this problem?
Any help is much appreciated.