TestFlight has been in lockdown mode for a while, and I've had plenty of opportunities to put the service through its paces before they opened their doors to everybody this evening. I'll just say that TestFlight is incredibly convenient for installing beta applications on the fly - there's nothing more annoying than unzipping iPad or iPhone files, dragging them to iTunes, syncing, and it doing all over again when updates are distributed. TestFlight simply has you log into their site via your iPad or iPhone, tap a button, and you can browse through the beta apps developers have subscribed you to for immediate downloads. It's so much easier for developers to submit one beta to TestFlight than to package their app and distribute it to dozens of emails, and it's much easier on the tester to be updated and install updates. It's completely Apple worthy, and if you're a developer we encourage you to sign up for the service so you can focus on creating content instead of worrying about beta distribution.
TechCrunch notes that the service is free to developers, while a paid Enterprise version is in the works that will allow for licensed versions of apps to be passed through the airwaves (presumably for corporate/in-house application use).