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,
David R.
Dear David,
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