Throtteling to KEEP enough bandwidth, not reduce congestion
They are throttling not only to leave room for their own Video Store but they might eventually offer true TVoIP in HD. That is why they installed Fiber to the curb (to my area at least). This was mentioned to me by a Bell tech.
Thing is, P2P's relative usage is shrinking. Right now, it makes up about 50% of all traffic (down from 60% a while back). As streaming video grows far faster, P2P's share of the bandwidth pie is shrinking rapidly. By 2012, streaming video such as YouTube will make up 50% of net traffic (I believe that was in the Ars article), making P2P less important.