I've noticed a drop-off too. I'm using Gnucleus which gives you more info than LimeWire, but I can't really see anything too unusual. What I'm seeing mostly is a drop-off in the number search results VS. searches. Usually this kind of thing happens when a more popular client makes a change (or has a bug) in its handling of things. In such cases I like to hope it's usually accidental and will be fixed. Unfortunately, as the number of different peers increases this kind of problem may become more likely.

A ddos attack would have to come in the form of a flood of search, ping, or pong packets (etc.) since completely bad packets are usually just dropped, but while I'm seeing a lot of those, it's not unusually high (at least to my eyes).