So you've got a friend who has written an iOS app and he or she wants you to beta test it. (Hint: if you're reading this, I'm that friend!) What do you need to do?
You need to provide that friend with your iOS device's "Unique Device ID" or UDID. Your friend can then generate a certificate that tells your device that it is allowed to run the beta app. You can drop this certificate and the app itself on iTunes and iTunes can then install the app on your device. All well and good but UDID's are long numerical strings and are a pain to find and type in anywhere. Apple provides an interface for getting them but it works a bit unusually and I thought it was worth writing some notes.
Here's an image of the Apple instructions. Everything you need to know is here but you have to read it carefully.
So here are step-by-step instructions:
If you have Wi-Fi Sync enabled your device should already be in the sidebar. If you don't have Wi-Fi Sync turned on then you'll need to connect the device with a cable.
Select the device in the sidebar and make sure you have the Summary Tab displayed. You'll probably see the Serial Number displayed.
Click the Serial Number itself (or the text label that says "Serial Number") and it will toggle to displaying Identifier (UDID): and the actual UDID string.
OK, this is the weird part. Even though you have no text selected if you hit Ctrl-C right now iTunes will copy the UDID onto the clipboard. You can't select the text in any way (since a click will change the display back to Serial Number) but trust me, this should work.
Paste the UDID from the clipboard into an email and send it to your developer friend. He or she will thank you!