Dear Aunt TUAW,
There's a tiny spider running around just behind the glass of my iMac. Hopefully it won't be cause for a service call!
[The photo is attached at the end of this post.]
Your loving nephew,
Obviously your Mac isn't running the Robot Exclusion Standard that keeps spiders off of it.
That's a little Auntie geek humor.
All in all, it's probably harmless if a little unsightly. It probably got in through one of the ports or the speaker grill. Give it a few days and it will either find its way out or fall to the bottom, where it will go peacefully to eternal sleep and decompose to dust.
Auntie consulted with Josh Carr of Denver-based MacWorks, who demurred. "If someone brought this in to me, first thing I'd do is disassemble the iMac to take a look around to see if the spider had made itself at home and laid eggs. If the nest is on the board, it could potentially short out."
He mentions that the iMac glass can be removed with a couple of suction cups, but that's something that freaks out Auntie's sensibilities. She'd more likely than not break the glass while trying to lay it down on the ground. Your mileage will vary.
In any case, you may want to get it looked at by an Apple Authorized Service Provider but things are probably going to be just fine without.
Hugs and kissies,