I've been linking to this for some time in answer to questions from the forums, but thought it was about time to put up a blog entry so they'd be more discoverable. The master pages and themes forum often gets questions about how to dynamically change the Theme of Master Page, perhaps based upon user criteria. So I created a web site that does both, using the Profile object to store a user selectable Theme and Master Page.
Feel free to download them. There are two versions, one in VB and one in C#.