But will it replace E-mail? Not any more than E-mail has replaced phone calls. Like a phone call, IM is a superior medium, compared with E-mail, for getting questions answered quickly or having collaborative discussions, but E-mail provides capabilities that real-time communication can't emulate.
E-mail uniquely provides a forum for creating, receiving, and storing structured ideas. If IM is the equivalent of brainstorming, then E-mail is the next step of outline creation. Both are critical to developing a full thesis.
Put simply, E-mail has evolved to provide a variety of other purposes that make it indispensable. Many people use their in-box as a to-do list, deleting action-item mail only upon completion--difficult to do with a transitory IM chat box. Information archiving is another critical function of electronic mail. It's certainly possible to save instant messaging conversations, but who wants to read through a dialogue to piece together the key message? It's the difference between reading a screenplay and a novel--both provide similar content, but the latter is a far more frequent choice for reading.
But these E-mail advantages don't mean you should dispose of instant messaging. E-mail can never replicate the speed at which inchoate ideas can be honed into tightly defined action plans by two people IMing.
Similarly, the very intrusiveness of the IM pop-up box provides a way to get answers to questions that may otherwise languish in a co-worker's in-box. Of course, you have to hope the answer you get is one you can remember, because trying to save IMs is the equivalent of building an address book using Post-It notes.
Luckily, the software community has been quick to recognize the complementary nature of both collaboration platforms. Some mail clients already offer the as-yet rarely used option of chatting, and instant messaging provides better and better archival services. So you're just going have to get used to the annoying pop-ups from your buddies asking you if you want to hit the pizza joint for lunch in the middle of your executive presentation. And if you haven't learned to deftly drop emoticons into your daily writing, get started because there's no stopping the IM surge. C U L8r. ;-)