A week or two ago, Jeffrey Sambells had a blog post about using Xcode's Enterprise distribution for ad hoc distribution. It's a great idea that a lot of developers saw had great potential, but it had several manual steps.
Since that post, Hunter Hillegas of Hanchor, LLC and developer of Vegas Mate, has created an application to automate the entire process. The app is called Beta Builder, and it's free. If you do much ad hoc distribution, you should check it out.