Be careful when reading your serial number, as 0 and O are easily confused. All letters should be upper-case.
Kindle Hardware
Your Kindle's serial number is printed on the box that it was packaged in when you received it. If you do not have the box from which to get the serial number:
From your Kindle's home screen, navigate to the "Settings" page.
Type "411" (without the quotes). A super secret screen should now appear.
Submit the 16-digit Kindle case-sensitive serial number and personal email address in the form below to instantly calculate the PID number.
iPhone/iPod Touch (40-character ID)
If you're on a Mac (vía System Profiler)
Connect your iPhone or iPod Touch to your computer
Click on the “Apple” symbol in the left upper corner of your screen
Click “About this Mac” (-> a small window opens)
Click the “More Info” button
Select Hardware -> USB in the left menu bar
Click on “USB High-Speed Bus in the USB Device Tree and select “iPhone” or “iPod”
Copy the 40 digit number (hex format)
If you're on a PC/Mac:
Connect your iPhone or iPod Touch to your computer
Start iTunes
Click on your device name (xxx’s iPhone) under the left menu bar “DEVICES”
Click on Summary
Click on “Serial Number” (yes, you have to click literally ON the word serial number, like shown below)