I set up web distribution for ad hoc builds today. I found this: http://github.com/wbroek/IPA-Distrubution

Check their example.php -- you still need to manage a provisioning profile with device ids, but from there you just do Build and Archive and put the .ipa and .mobileprovision file alongside the above scripts. Their code extracts application info, including thumbnail, and builds a nice iPhone-formatted HTML interface. Any iOS4 device can visit your URL and install the app (assuming their UDID is in your profile).

It's using a new feature in iOS 4. Technically, you don't need this service to do the direct web-to-device ad-hoc installation, but I'm in the beta and it's a great product that makes things easy and smooth.