I am working on CS5, and apparently do not have the "Unite" command in Pathfinder. I did try the Merge command, which I think did a similar thing -- it became a big red filled shape. However, I lost the stroke, and in Outline view all of the circles were still there.
I have not worked with Pathfinder, and I need to in order to understand it better. You may have the solution with Pathfinder, I just don't know how to do it yet. I'm using Lynda.com to help me become more fluent.
Thanks!