This is a set of (at least at the moment) shell scripts that manage a process whereby code can be modified from and uploaded to Dropbox, at which point a Mac running these scripts will notice the Dropox changes, check those changes into git, build a new iOS executable from the git changes, push that executable to a web page, and send the user a push notification to come get it.
It's MIT licensed, so feel free to try it out and send me feedback or updates via fork requests. I find it useful, and I hope other people will, too.
I'm currently using BetaBuilder to generate the ipa file, and using the Boxcar API to send push notifications.
I've done some cleanup so it should be conceivable that it should work on someone else's machine (although, as far as I know, so far it never has).
I think what I've decided to do long term is to make a Mac MenuBar (NSStatusItem) based App and use NSTimer instead of running it from cron, and then work on moving a piece of it at a time from bash to Cocoa. Then I'll make an iOS app to simplify what you have to do on the iOS side.
I just (1 July 2011) pushed a version that has preliminary TestFlight support. It still needs work. For example, it doesn't check return codes from the TestFlight API, so XCAB doesn't know whether the upload worked or not, and right now TestFlight is updated in addition to BetaBuilder instead of as an alternative to BetaBuilder (it seems to me that people would most likely use either one or the other but not both). I will be working on it more.
Now (August 10), I've switched to using the command-line options from the Mac App Store BetaBuilder, so you'll need to buy it from http://itunes.apple.com/us/app/betabuilder-for-ios-apps/id415348946?mt=12 to continue to use updated versions of XCAB (or switch to using testflight).