That integration is DBus and Telepathy at work. Telepathy provides a uniform system for creating communication "tubes" and the IM clients are built on top of that. DBus allows programs to send messages to one another in a standard way, and Telepathy exposes its workings via DBus. So any application can fire an event to request a phone call begin, an IM be started or sent, or can monitor the workings of any of those (to the extent that the applications expose things to DBus, of course).

This kind of thing is why a Linux based system such as Maemo or Meego is ultimately more powerful than an app platform like Android/Davlik or iOS. The contacts page can initiate calls, emails, im's, or any other communications, as well as map pages, web sessions, or whatever you can come up with.

This integration is actually the feature that attracted me to the phone in the first place, and it has not disappointed. Android does have nice integration, and I'm sure they provide very good api's for doing phone functions. But DBus lets ANY application interoperate with ANY OTHER application as long as they both use DBUS. And the best part is that this is standard Linux (DBus is everywhere, Telepathy is a little newer) so there is plenty of info on how to do it, support in every language, and a long history of this stuff being tested and working.