If not finding any specific chat corpus, you might look into dialogue corpora in general, e.g. you can find corpora on movie dialogues (audio files and transcriptions).

Hi

Here is what you asked for :

http://www.isi.edu/natural-language/people/hovy/papers/05ACL-email_thread_summ.pdf

enjoy !\

and this one :

http://www.aclweb.org/anthology/D08-1081

also  check out this:

http://www.aclweb.org/anthology/D08-1081