As you may know, we use EventMachine a lot at work. I've had a bit more time to get familiar with EventMachine then some of the other guys here and was asked to do a presentation to get everyone on the same page.
Not knowing where to start, I started from, what I think is, the start. So, if you're interested, you can check out the EventMachine Presentation or the EventMachine Introduction I wrote to support the presentation. (Note, it looks like Evince doesn't draw the background correctly in the presentation so it might look a little strange but readable).
There is a lot more stuff I could add to the introduction, I've just run into time constraints at this point. Hopefully, I'll be able to expand the introduction and get into some of the non-core EM software like EM-HTTP-Request and the EM AMQP library.
Let me know if you have any questions or comments.