Increase in traffic to/from DSL subs since August?

Greetings. Another independent ISP operator and I have noticed a pretty significant increase in traffic to and from our broadband (DSL) subscribers since August. It's been a fairly steady uptick, at least in my case, resulting in a doubling of overall average traffic to/from these folks since then. Have others seen a similar trend? Any thoughts as to what the cause may be? Our best guess a virus/worm, possibly being used as a spam relay or other proxy at this point... Many thanks, -- Jared