Do not...I repeat...DO NOT try to trick your computer guy into installing software on your computer that you know and he knows you're not supposed to have.
Don't try the innocent request approach. It's not cute when he knows your department all got bitched out for abuse of this particular software.
Don't try the "oh, you didn't get the memo saying it was ok now" approach. This is insulting to the max when the IT guy still remembers how your boss was breathing down the IT department's neck to block this program from working.
The debating like a three year old when there really is no debate method is really demeaning to everyone involved. Just don't go there.
The empty threat method? Yeah, how'd you like your boss to know what you really do at work all day?
It's not that the computer guys are spiteful bastards who hate you (well, they may be.) It's just that we don't like having to explain to someone in a position to make our lives uncomfortable why Suzy Smith has AIM installed on her machine when it shouldn't be there and only an admin has the ability to put it there.
Oh, and when the computer guy tells you "have your boss email me telling me it's ok and I'll install it ASAP," debate has just been permanently closed and you have just been told (politely) to STFU. Continuing past this point is just going to get the IT person pissed at you and will probably result in any further service requests you make being put on a very low priority.