I would love to have an IM client that was tightly integrated with Acrobat.com.  Some use cases where I see this being useful:

1) File Organizer: there is a buddy list that is pre-populated with everyone I've interacted with on the site.  From this buddy list, it says if a user is online on Acrobat.com or not.  If they are online, I have the ability to initiate a text/voice/video chat with them or optionally invite them into a ConnectNow meeting or to view a document with me.  The experience from the File Organizer view would be similar to that in Gmail Chat.

2) Buzzword: While Viewing a Buzzword Document, I would see a list of my collaborators as I do today, but there would additionally be presence information and if the person was online I could initiate a chat with them and invite them to view the document with me so I could clarify any questions I had with them.  I would also love to receive automatic notifications such that if I am on the site, and someone comes to view or edit one of my documents I am notified so I can go with them.

3) Connect Now: I am in a meeting room, but I'm waiting for Jane to arrive to the meeting.  She is late, but she is my buddy list which I can pull into my room.  I send her a chat message which includes a link to the room, and she clicks it to join.  After a few minutes we realize we need Sergio to join so we send him a chat message from our buddy list seeing if he is available to join.  He is so we send him a link to the URL in chat and he joins the meeting.  Alternately, I forgot that I scheduled a meeting in my room.  Julie joins the meeting but I'm not there.  I get a notification that "Julie has entered your meeting room" with a link to jump in.

What would make this even more useful was porting this chat client to AIR such that I could keep it open on my desktop, set status (invisible, away, etc.), and then people could IM me from within acrobat.com but I don't need to be there monitoring items 24-7.  The system would let me know when I needed to take action on something like join a meeting, etc. and I could be always available (or as available as you wanted to be) to collaborate with others on acrobat.com documents/meetings.