GMail Filter-to-Forward
I have been playing around with the forwarding capability. Since you
are not limited to simply forwarding all mail, but you can set filters
to forward mail conditionally to multiple accounts, this is a very
powerful tool.
Caution should be excercised when filtering-to-forward when forwarding
to third parties, I think. Unexpected consequences etc. :)
You need to play around with it a little to see how it works.