How about this from the Herman Trend Alert – Use IM to Reduce Interruption (sorry, no link).
Most people believe using “instant messaging” (IM) software to chat at work leads to an increase in disruption. In fact, a study published recently by researchers at Ohio State University and University of California, Irvine found that workers who used IM on the job reported fewer interruptions than their colleagues who did not.
The research showed that IM is often used as a substitute for other, more disruptive forms of communication such as the telephone, email, and face-to-face conversations and thus it actually leads to an increase in productivity. Dr. R. Kelly Garrett of Ohio State and James N. Danziger of UCLA, Irvine co-authored the study.
Those results just seem like a stretch to me, but I am not an IM guy. When I do use it, I feel more obligated to respond quickly – even more so than with email.