This is called an "ad hoc distribution." First, the developer will need your iPhone's UDID. He'll have to create a special "provisioning profile" so that you can install it on your phone. This profile will be provided to you as a file you will have to install. To find your UDID, sync with iTunes and, looking at your device information, click on the Serial Number. Your UDID will appear.
Once the developer has that info, he'll send you the app and a provisioning profile. So how do you install it? Here's what you do. The assumption is that he'll provide you with a .ZIP file for the app.
Installing the Application
- Drag-and-drop the .mobileprovision file onto Library / Applications in iTunes. On the Mac, you just drag the file to the iTunes icon in your dock.
- Extract the .zip file. I'm going to assume you know how to do this. In Windows Vista, it's been widely reported that Vista corrupts the application if you use its built-in-command. I'm unclear if this occurs on 7, but there's no reason to not use a separate (read: better) zip utility anyway. I'd recommend the open-source 7-Zip if you don't want to buy one, such as WinRAR.
- Find the .app folder, which will usually be AppName.app.
- Drag-and-drop the entire .app folder onto Library / Applications in iTunes. Once again, on the Mac, you can just drag it onto the iTunes icon in your dock.
- The application should now show up in Library / Applications. Note that it will have the default App Store icon.
- In iTunes, select the iPhone, select the Application tab, and verify that the new application is checked.
- Sync your Device.
- The most difficult part of this is removing the provisioning profile, not because it's that difficult, but because unlike what you might assume, removing it from the iPhone isn't enough. It will simply be reinstalled by iTunes.
- To remove the provisioning profile from your iPhone, go into Settings, General, and scroll down. Just above Reset you will see Profiles. Click through, and remove the profile from your device.
- Then, you have to find the file on your computer, to delete it from there. Naturally, the location is different in each OS, just to make life interesting.
- On MacOS X /Users/username/Library/MobileDevice
- On Windows XP /Documents and Settings/username/Application Data/Apple Computer/MobileDevice/
- On Windows Vista /Users/username/AppData/Roaming/Apple Computer/MobileDevice/
- On Windows 7, I would assume it's the same as Vista, although I have not confirmed it
- You can then delete the app from the iPhone in the same way you delete any app: press on an icon until the icons start to shake, then tap the X in the upper left hand corner of the appropriate app.
- Finally, to be absolutely sure it's gone, go to iTunes, under Library / Applications, and remove the app from there, as well (right click on the icon and delete it).