This is taken from 1 ISP's FAQ, but other ISP's are following suit with packet-shaping technology. ISP's are not saying they use this, but it's becoming more common, as you see more and more posts on slowing speeds with p2p apps.
7. What are P2P ports?
Peer-to-Peer (P2P) ports are used by file sharing system such as KaZaA, Morpheus, Grokster, Gnutella, LimeWire, WinMX, or eDonkey2000 to download large amounts of data.
A small number of heavy P2P users can utilise a significant proportion of available bandwidth. This can result in a significantly degraded quality of service for other users.
9.1 Does TalkTalk allow P2P downloading?
TalkTalk allows the use of P2P software, however, it does not condone its use where subsequent file sharing breaches applicable local laws and regulations. This includes, but is not limited to, those principles of law which protect against compromise of copyrights, trade secrets, proprietary information and other intellectual property rights, libel or defamation of character, invasion of privacy, tortuous interference.
9.2 Does TalkTalk block P2P ports?
No, TalkTalk broadband does not block ports used by P2P file sharing systems.
9.3 Does TalkTalk broadband prioritise traffic generated by P2P software?
TalkTalk broadband may use network controls to prioritise traffic generated by web browsing, email and music downloads over that generated by with P2P file sharing systems where it is found that it affects the bandwidth of other broadband users.
9.4 Why does TalkTalk put network controls in place with P2P networks?
TalkTalk owes a duty to all our Broadband users to both preserve network integrity and avoid network degradation. If, in our reasonable opinion, we believe P2P usage has, or may adversely affect, such network integrity or may cause network degradation for the majority of users we may control the transmission speed of selected ports.
9.5 Does TalkTalk use network controls to affect gaming or other legitimate download services?
The techniques used to control the network should not affect legitimate download, chat, instant messaging or gaming services.
Now time to lock this discussion.