Stinky Dad’s Search Comment plugin at Frorse’s plugins (via) was apparently written for WP 1.5, but it seems to work just fine with 2.0.5, too. The results blend smoothly among the results of the normal, posts-only search, which was exactly what I was looking for.
Funnily, as I tried Dan Cameron’s Search Everything, which is said to be 2.0 and even 2.1 compatible, that didn’t work for me.