Lion Kimbro noted (over at RiotsInFrance) an experience of there were a few points, where, on the "Recent Visitors page," I could see that you, me, and Helmut were all here at the same exact time. It's like: "Yrrrrgnh!" Just: total frustration! Because I know you're there, I know you're interested, I know we'd have an interesting conversation. These seems like a Wiki And Irc Group Forming opportunity...
For Team Work centered around a single Team Wiki, you could just have the best practice of having an IRC space that everyone logs into at the start of the day, etc.
problems with this? annoyance over threads that don't relate to you... though if your IRC client has a good sound control that can be set to beep only if someone sends you a private/direct message, then maybe that's not an issue...
But as Wiki Proliferation spreads, this is not realistic. So you want a way to make it easy to ask someone who's "in" the same space at the moment whether they'd be open to some Real Time communication....
you could "promote" a certain IRC channel on every page of your wiki (whether it's a channel specific to the site, or maybe just a related-topic channel...), but how many people are going to bother going there, just in case someone wants to ping them?
Lion Kimbro suggests at WikiAndIrc showing (Java Applet?) the last few messages in the "associated" IRC channel at the top of each wiki page - so maybe you could say "yo Charlie!" and they'd notice it up there. But I wouldn't count on it.
do many people even put their Instant Messaging handles online? Or are they avoiding the Spam Wars? If their local person-page (linked from Recent Visitors) links to a page holding their handle, then maybe they're just 3 clicks away...