You can only authorize 5 computers at a time. It's based off some hash file the computer makes from all the hardware, kinda like the windows xp thing. If they replace any parts or the os, the hash file is different. You can deauthorize all the computers once a year without having to call Apple. If you just deauthorize the computer before sending it in, if you can, then it just removes the hash file off the iTunes server and you'll be able to reauthorize it when you get it back.