A great deal is said about P2P in the media, and the government is starting to truly weigh in as P2P has been in the courts, the subject of FCC and FTC hearings, and Congressional action is on the table now. Unfortunately, nobody who knows how much P2P traffic there is has released that information. There are dozens of conflicting reports, but the latest large study (by Sprint in 2005) put that number at 6%, with regular Web traffic taking 50% of the pipe. These numbers would not seem to indicate a large enough problem to warrant legislation, but this wouldn't be the first time the government used skewed, old, or just plain wrong data to justify policy.
Of course, P2P has been in the courts lately, as TorrentSpy was handed a judgement of $111 million after refusing to turn over internal documents. In fact, you don't even have to host files or distribute copyrighted materials to draw a lawsuit as the creator of a popular BitTorrent search engine is finding out. But the biggest problem with P2P is that it provides a fertile playground for malware, as P2P is a somewhat common attack vector.
Fake P2P media files lead to adware attack (Secure Computing)
At my old job, we didn't worry too much about P2P. Though we were in education, we did not have dorms, so student use was either from our computers (locked down) or their personal laptops (outside the firewall and using a Clean Access server). We had a Packeteer that we used to draw the flow down to a trickle, not blocking ports as the software reconfigures itself if it finds a blocked port. At the university where I now work, we have students living on campus in dorms, so P2P traffic is a much larger concern for us. What do you do with P2P traffic on your network?