Various people have integrated various different chat or instant
messaging tools into their OpenACS sites, including Jabber, IRC, and
NFC Chat. But AFAIK none of those have been rolled back into the
standard OpenACS distribution.
It's been discussed here in the past, here are 3 threads from
Feb. 2004,
June 2003, and
March 2003.
Searching the forums will definitely find more.