In order to make it easier for those arriving at this post, here are the actual instructions (straight from the blog post linked from the accepted answer).
There has been some talk on twitter about how to create your own IPA file for your iPhone app, so I thought I would give the instructions that I have used to build an IPA before. Enjoy.
- Create a folder on your desktop called “working”. Open that and create another folder inside of it called “Payload” (case-sensitive)
- Move your iTunesArtwork file into the “working” folder and your .app into the Payload folder.
- Open Terminal and run the following command: chmod -R 775 ~/Desktop/working/Payload
- Go into your ProgName.app folder within Payload.
- Double-click the Info.plist file. Make sure there is a item called: SignerIdentity with a value of: Apple iPhone OS Application Signing. If there is not, add it.
- Zip it all up. Zip the iTunesArtwork and Payload folder. (So zip up what is inside of the working folder)
- Rename the zip file to have the name you want, and the extension of ipa.
- Double click to install with iTunes