I recall a number of people asking for this functionality, so I thought I'd create a plugin for it and share it here! Announcing Premium Themes!
This integrates with the Supporter plugin here to allow you to select which themes are only available to paid Supporters. Pretty simple, but it sure beats manually enabling themes for supporters.
Check it out here:
Note, if someone is really needing this to integrate with the Upgrades plugin you can post it to the jobs section here and I will take it up.