One feature that I’ve totally missed on iPhone4, was the ability to download an .ipa other the air. You can now easily download an app if you specify a link. The requirement, is that your application need to have a valid certificate for this device.
So it’s not possible to download your app on any device, as it will require Apple signature, but it can be done for AdHoc release for instance (developer releases). But the management of the certificate, device id, and so on is usually a pain.
There is a simple service that do it very well: diawi . Just upload your zip or ipa, and then you’ll get back a link that you can forward to all your tester, with an optional password
Note: this works only on iOS4….