The reason why I like IRC so much more than IM (even if I seldom use it nowadays), is that you can hang out there just skimming through the channel's log, but nonetheless at the end of the day you'll end up learning something.
That doesn't happen with one2one IMs.
On a physical parallel, you can think of going down to a pub or agora as opposed to making phone calls from one's own living room.
IM, as phone, turns off the "noise" completely (except from the occasional SPAM, of course), while IRC lets you manage (either cognitively or technologically) the squelch.
IRC, as muds, pubs, agoras tend to allow or even push serendipity at the cost of a higher cognitive load (but of course, this could be managed through ambient knowledge...).
IM, as phone, audio/video conferences, letters (even if this is totally different media), and in general all forms of one2one communication deliberately deny serendipity, while focusing on the main stream of content.
I don't think either tool is objectively better than the other: they're just tools. But maybe your feeling about them can be significative of your way to handle communication and knowledge.
That's why I think that a nice addition to certain kinds of 121 (one-to-one) conversations is a side element in the interface that shows the same informations contextually to both the users.
Something like a cloud based on the common interests (or based on conversation keywords?), with links triggering something like, uh, Wikipedia pages? Digg links? Buh, I think it's not so important. It's important instead the relevancy of the linked keywords. :P
i think using that both sides together in the same app. can enhance the value of the experience. in a collaborative product for the financial community ( Reuters Messaging ) topic based ,persistent chatrooms were added sideby side with 121 or group IM
It's also quite interesting, thinking about it again, that it isn't a technlological limit... it's a matter of "how"...
How could we do that in a natural way, without any clutter and using current technologies?