[logical vs physical topology]
My article is a bit sloppy on this point. One way you can look at a topology is the physical fact of how the bits move from machine to machine, the details of the routing. That view can be nonsensical; for instance all Internet systems are in some sense decentralized because that's the way Internet routing works. So it's often useful to look more at the logical structure of the system. oreilly.com is logically a single centralized web site, even if physically the site itself is organized as a ring and your web browser's packets get there via a decentralized system.

[centralized+decentralized connections]
In the centralized+decentralized system I describe in the article, what I mean is that you have a group of decentralized hosts acting as centralized servers to leaf clients. I recommend taking a look at the KaZaA/FastTrack design and its use of supernodes as a good example of this.