Went hunting around for some chat room corpora today–I though I’d find tons and tons but really just turned up one resource.
But it’s a big one: over 30 billion words across 47,860 English language news groups from Oct 2005 to Jan 2011. Posts that are not in English are pulled out and the people are anonymized.
Read more about it here at the Westbury Lab’s site or go get the data, which is stored on Amazon (150 MB chunks adding up to 40 GB).
[Update 2/29/2012: Here’s some more stuff though it looks like the counts are kind of small: http://caw2.barcelonamedia.org/node/7; also check out the comments for another corpus to check out.]
Corpus linguistics 
Another chat room corpus: Micha Elsner collected text from chat rooms and labeled each utterance with conversation threads. Check out IRC Chat Data at http://www.cs.brown.edu/~melsner/