As a Washington resident, I can tell you that Melissa Ethridge is an almost a permanent attaachedment to every march--she has been on stage at the last three, The WTO/World Bank march, the Gay/Lesbian thing, and, though I am not sure what songs she plays, I'll bet you could find them at one of her fan websites.

Musicians who play often simple do whatever they do, whether it has related content or not--there is drumming and that sort of thing, and periodic chanting, but there is not the kind of singing of issue related songs that there used to be.

The main thing is, if you have cable, and get any of the CSPAN channels (here inside the Beltway, we get three of them) check them out, because they tape hours of march related stuff and broadcast it from time to time. They basically turn the camera on and point, so you get a much better feel for what is happening than from the news.