Marc Andreessen: cofounded Netscape and then OpsWare and Ning and now is a VC. Serial to the max.

Ben Horowitz: Opsware and then VC. A Morten Lund character.

Bill Gates: already addressed in both the article and another comment

Silvio Berlusconi: I don't know much about his business history, but a quick glance at his Wikipedia page shows he started multiple businesses. Definitely not a one-business-and-retire type.

Michael Bloomberg: Ok, that one may be an example.

Niklas Zennström: Skype, Kazaa, Joltid, Joost, and now a VC. Serial to the max.

With people like Gates you could also argue that they create 'new' startups through their existing organisations. Why wouldn't you if you have a brand, distribution, etc that were hard-won?

Also, Gates went on to set up the Bill and Melinda Gates Foundation, which is arguably a different kind of startup.

Even Bloomberg is almost a serial entrepreneur. Effective, memorable politicians must be to some extent entrepreneurial--you have to believe in your own ability to create change and to unify people behind your cause.