A little while back, a guy with child porn was caught because he DLed from Kazaa. This got me wondering if the piggies do that kind of thing, are they monitoring the networks for (for example) Urbanski. Of course, having any book is not illegal but I wouldn't like to appear on a potential terrorist list.

How would they have found out what the paederast was DLing anyway? Do they put the hot stuff on a machine that is monitored for IP traffic on that port?

Do they put the hot stuff on a machine that is monitored for IP traffic on that port? It's well possible that they do it that way. I can also think of them searching for relevant keywords and having a closer look at the sources of these files. Since your P2P client (on the most popular P2P systems) needs the IP address of the machine on the other end identifying you isn't too hard. I doubt that something like Urbanski can get you busted, though, because that one is already too wide-spread and the chemistry and technology of explosives doesn't have a reputation as bad as child porn.