Not really "one time" unless you plan on using your phone for a lifetime. They get you for $36 everytime you buy a phone. I am actually surprised that congress hasnt had hearings on this yet. It is a required hidden fee.
The activation fee is not a "hidden fee". It is listed on the terms and agreement that you sign when you upgrade. I have yet to pay it, though. I always call AT&T and have them remove the fee once I see it on my billing statement. While this isn't something they have to do, I find that if you are nice when you call them, they are more than willing to remove the fee.