I've almost completed a script to notify the user of updates to private themes and prompt them to auto-update.
It's based on this script for plugins...
I don’t know if anyone wants to help convert this plugin into a theme updater but i’ve hacked it a bit and got as far as displaying the fact there is an update in the sidebar (so it is checking the versions).
Anyone who wants take it further here is the code….
I mainly just did a find a replace on “plugin” to “theme” and change the file it looks at for the version numbers. It just needs to display the update details and download link. I think the plugin and theme api is very similar it is just a matter of finding the right terms.