As is often the case now in the Age of the Internets, I'll get an idea for a post, do a quick Google search on the topic, and find an article where someone has said everything (or almost everything) I wanna say about it. So anyway, here's a link to the article (on, in this case, improvisation in rock music), from the excellent Perfect Sound Forever website:
I also found some useful information from the following text; I of course take exception to the characterization of the Grateful Dead's improvisations as "ponderous", but I gotta give some respect to any author that's able to discuss the Dead, John Coltrane, and Black Flag in the space of two pages and say something significant about each one.
From "The Path to Freedom" in Free Jazz and Free Improvisation by Todd S. Jenkins
For those of you who still have no idea of what the hell these guys are talking about, here's of few of my favorite examples of rock improvisation, courtesy of YouTube:
Johnny B. Goode (Live in Berkeley, CA 5/30/70) - Jimi Hendrix