Om Malik offers a guest post by Jesse Kopelman:
The business model I envision for Skype could be generalized as Centrex service. The idea behind Centrex is you want the benefits of a PBX but not the cost of buying the equipment and the hassle of maintaining it. So, you get a company (usually your local phone company) to do these things for you and send you a monthly bill. What are the benefits of a PBX? The most common are things like voicemail, being able call the other people on the PBX for free, and the cost savings of being able to efficiently share a smaller number of lines than users. Funny how those things are pretty much what you get from using Skype. What is more, most PBX/Centrex users have fancy phones that let them have multiple simultaneous calls and make it easy to conference calls together functionality found in the Skype client.
Skype goes beyond the traditional PBX features with its support for IM and ability to share contact information with other applications. These features are exactly the selling points of the newest software based PBX systems. The IM thing especially, as it ties into something called presence. The thing that gives Skype a huge advantage over say the latest Avaya PBX is that you automatically get the advantage of having millions of existing users on the same PBX as you and it costs nothing to add more. Meanwhile, with a traditional PBX/Centrex solution you are going to be paying a monthly fee for each and every user and you dont even want to think about what it would cost to have 3 million simultaneous users.