hidden a bit hard to find on the Apple iOS developer website this small guide on Distributing Enterprise Applications for iOS 4 Devices can be found. Amongst other things this guide explains how to install your app and provisioning profile wirelessly.
Turns out that while the guide is for Enterprise distribution, wireless distribution also works for normal ad-hoc apps. Jeffrey Sambells wrote a short article explaining the basic requirements for wireless Ad-Hoc distribution. It involves special links in html and a xml plist file describing your IPA archive. Jeffrey even created a php script to automate much of the work.
Even more simpler is the recently released app called iOS Beta Builder. After you Build & Archive (you do archive right?) simply select the generated IPA and iOS Beta Builder creates the required webpage and manifest xml. Upload to the server and your app is ready for download. No custom scripts needed.
The Hockey framework also uses wireless distribution, and wraps it to provide an automatic update functionality for iOS apps (not unlike Sparkle on the desktop). Using Hockey your ad-hoc users will be notified of new builds as soon as they are available.
Still in private beta is TestFlight. I’ve only seen a short video, but it appears to be an all integrated environment that offers wireless ad-hoc distribution with review and feedback opportunities for testers.
As frustrated as ad-hoc distribution might have been in the past, Apple is certainly improving the process. The third party tools mentioned above really make ad-hoc distribution as simple as possible.
(Like before, wireless ad-hoc distributions still require each device to be included in the provisioning profile).