We just sold the first two books of a potential trilogy. When I sent my partial to my agent, I also included a short "series pitch" I had done--brief, one- or two-sentence descriptions of each of the three books--and told her to do whatever she felt appropriate. She looked at them and said, "We'll just include this with the partial and let them respond how they want." They responded by asking for Books 1 and 2. I'm very pleased that we decided to go this way.
Now, as for that "series pitch" thing? I have no idea if that's how it's done or not--but I had the ideas and I wanted my agent to see them, at least, so I wrote them up the way that seemed to make the most sense to me.
And lastly, I guess I should say that this was sent to the house that had already published my first book. I'm sure things would have been different for a first-timer, or for a new house.
(edited b/c apparently I've forgotten how to use verbs.)