During one of the OCS 2007 R2 presentations at TechEd, the presenter talked about a firefighting organisation in the UK that had made great use of instant messaging to deal with a chemical spill.
It just so happens that one of the members of the Unified Communications group that I'm a member of on LinkedIn, Mike England, posted the details of this story to the group. It makes interesting reading. I can imagine instant messaging being much more useful than trying to hear someone on the other end of the phone whilst there are sirens and other loud noises all around you. You can find the story here.