Please check this:
http://www.i-m-d.net/scripts/msg/ It's great and freeware.
It can be implemented in the buttonline above the teksteditor (profile-pm-edit-home-quote), but it needs some html/xhtml to do that.
This script can be modified to communicate with members on the forum. It can be displayed as a java popup window, there could be a wav. in it to let members know that there is an IM.
And maybe a block cookie code can make this work hand in hand with the "who's online" notition.
