Hi thucydides,
That's several questions
1) The idea which looked most promising to me can be found at
Credence: Thwarting P2P Pollution - sadly LimeWire didn't integrate it, and they didn't reply on my questions, so I don't know their reasons.
2) Injecting these false results is quite easy: Just reply falsely to messages. Gnutella is an open standard, so everyone can write a Gnutella client by following the specifications (
GDF - LimeWire), and they can just reply dishonestly. A client sees "someone wants X" and can answer with matching files, but it can also lie.
3) As far as I know the files range from just plain invalid results (which can't be downloaded) over porn spam to virusses. Just download one and run your virusscanner over it (but be careful not to execute it).
Happy sharing!