three displays + mirroring only two?

If you're just mirroring ... I THINK that you can get a splitter cable for the DVI. It'll just send the same signal to both monitors, and your computer will behave as if the two mirrored displays are a single display.

Disclaimer: I haven't done this, or looked into it, this is just what I remember.\

Edit: Google turned up this along with a whole bunch of expensive boxes. but this is what I had in mind.

 

I was trying to get this to work last night with the following setup:
Mac Pro (two Nvidia Cards)
2 - 23" Cinema Displays
1 - 50" Plasma

I wanted 1 CD to be mirrorred on the Plasma while the 2nd CD was an extension of the first CD. It was super late so I didn't have time to search last night but this morning I found this article on Apple's site, I believe this answers both our questions:

http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh1551.html

You want to read the "Tip:" part at the bottom.