Literally, "I ain't dead." A declaration of one's alive status, often used by bloggers and members of online communities who haven't been - or won't be - heard from for a while.
Originated in the comic fantasy "Discworld" books by Terry Pratchett, in which elderly witch Granny Weatherwax wears a scrawled "I ATEN'T DED" sign while having out-of-body experiences, to prevent passersby from getting the wrong idea and burying her.
I ATEN'T DED guys, I've just been at my auntie's house for a week and her Internet is crap. What'd I miss?