The first smartphone to receive the Skype treatment will be Nokia N97, starting somewhere in the third quarter of 2009, and other devices will follow.
In practice, this means that on the Nokia N97, you'll be able to see your Skype contacts and their presence in the phone's address book. You'll also be able to message them, as well as make skype-to-skype voice calls using 3G and WiFi.
This is especially handy for those international calls from conferences and similar events, when regular phone calls are expensive as hell, but WiFi is almost always freely available. Having Skype integrated with your phone's core functions means you'll spend less time fiddling with it to get it to work, which can make the difference between a very pleasant and a very annoying overall experience.