I think it would be pretty nice to see a dual monitor version of Aquarium (by far the best screen saver ever written).
It seems like more and more people are setting up dual monitors these days.
I recently purchased a GF4 which has built in dual monitor support and I am currently looking at a second monitor to add to it after successfully setting up a dual display at work (using 2 vid cards). I've found that once you have tried dual monitors for work or play, you will not want to go back to a boring, less productive single display setup.
As for Aquarium having to be written for dual monitors, I don't quite understand why that is needed. With every program I have tried at work, just stretching an application to the second window seems to work just fine.
Here is a link to the best multi-monitor resource site on the web:
http://www.realtimesoft.com/multimon/
Oh, and here are my dream monitors:
http://www.massmultiples.com/
http://www.9xmedia.com/
http://www.panoramtech.com/products/pv230.html
I wish someone could buy one for me at xmas but I doubt anyone will.
Anyway, keep up the great work on Aquarium!