How to install an IPA without iTunes using USB

Download and install the iPhone Configuration Utility from Apple.

You can get it for mac or windows here (look for "iPhone Configuration Utility")
http://www.apple.com/support/iphone/enterprise/

Here's some basic steps on how to use it:

To install an app using iPCU:

- Connect your iPhone/iPod to your computer via USB cable.
- Close iTunes, if it is open, before launching iPCU
- Launch iPCU. You should see your device appear in the “Devices” section on the left side of the window.
- (I didn't have to do this step... i think you can skip it if you've ever installed anything to your device before.) Add the provisioning profile (mobile provision) to the iPCU Library: Click on “Provisioning Profiles”, then drag the .mobileprovision file into the Provisioning Profiles pane.
- Add the application to the iPCU Library: Click on “Applications”, then drag the app into the Applications Pane. On Windows, the app will be a folder, named something like YourAppName.app. On Mac OS X, the app will be a single application file.
- Access your device: Click on your device in the list of devices on the left. You should see five tabs appear in the main window: “Summary”, “Configuration Profiles”, “Provisioning Profiles”, “Applications”, and “Console”.
- (I didn't have to do this step) Install the Provisioning Profile: Click on the “Provisioning Profiles” tab of your device. You should see your Ad Hoc Provisioning Profile in the list of profiles. In the “Install” column you should see either a button that says “Install” or a button that says “Remove” (if you’ve installed this profile previously).
- Install the app: Click on your device’s “Applications” tab. You should see your app in the list of applications. In the “Install” column you should see either a button that says “Install” or a button that says “Uninstall” (if you’ve already installed the app).
- If you've installed this app before, just click the button named "uninstall" then "install" to reinstall the app.
- If not, then just click the button named "install".