I ran across DebateGraph, (by way of this post by John Breslin) and it seems very exciting. I found it a little difficult to get my bearings at first, but then again I was in my usual mode of diving right in before doing anything too crazy like watching the introductory videos (Tip: when you get to editing a map, watch the helpful Dashboard text box and click the 'quick hints' link).

UPDATE: looks like it doesn't let you switch a debate from private to public. I've remade my original private one as a public one.

Here's a link to my public debategraph.

Here's a snapshot:

You can even embed a live copy of it within a web page, so that the debate can go on there. However, it does require quite a bit of screen area, and so is a little tricky for most blog themes.