Whoever sends me the most IMs before I get back gets a free coke. Ready, go.
I’ve never used this myself.
My usual:
Hey %n, I’m not in, but feel free to talk to this message if it makes you feel any better.
Playing Progress Quest. Can’t be bothered. Either that, or I’m not here.
My favorite:
%n, thank you for IM’ing me. Your IM is very important. Please stay on the line as the system traces your credit card number. This IM will cost US$2.99 per minute, rounded up to the nearest minute, minimum 5 minutes. Please hold.
I’m gargling with Lavoris. This could take hours. Please standby.
If I’m not back in 15 minutes, send a search party of scantily clad male models.
I’m currently being strange somewhere AWAY from my computer. Unexpected, but true.
I prefer the “bumper sticker” versions:
Athiesm is a non-prophet organization
Save the Whales; collect the whole set
Change is inevitable; except from a vending machine.
or, these play on words:
Intaxication: Euphoria over getting a refund, only to have it short lived when you realize it was your money to start with.
Ignoranus: A person who is both stupid and an ass hole.
Sarchasm: The gap between those who get the joke, and those who don’t.
My funny ones used to be
“Wurble wurble wurble” For when I was revising
“Not here. Gone to the Pub. Getting drunk. If I can type and/or read when I get back, you may just get a response” Self explanatory really.
“If you wish really really hard and clap your hands because you believe in me, I might just reappear”
“I’m looking at pretty pictures right now. If my marbles are still intact when I get back, you might get a coherent conversation, else, I’ll just sit and mutter quietly to myself.” This is for when I have a long day in the office.
“Ooooh! Changing Rooms is on!”
I use Jack Handy quotes as mine 
“If you saw two guys named Hambone and Flippy, which one would you think liked dolphins the most? I’d say Flippy, wouldn’t you? You’d be wrong, though. It’s Hambone.”
“When you die, if you get a choice between going to regular heaven or pie heaven, choose pie heaven. It might be a trick, but if it’s not, mmmmmmm, boy.”
“Contrary to what most people say, the most dangerous animal in the world is not the lion or the tiger or even the elephant. It’s a shark riding on an elephant’s back, just trampling and eating everything they see.”
Quite regularly, I will tack a disparaging remark about Hamadryad on the end of my away messages. Such as:
I’m not here. And Hama is a skank-ass
I’m out shopping. For Hama repellent.
I’m sleeping, because it makes Hama think I’m a fiend.
You know, I like to mention my friends as much as I can.
“Dropping off/ picking up heathens from the conformist factory” Depending on time of the day.
I used to use
‘I am God if the sea people!’
but people got confused so I stopped using it.
Interestingly enough, obesacantavit is my AIM screenname.
My favorite away message is “I’m not Haydn, I’m just in the Debussy. I’ll be Bach.”