Delta's D&D had an excellent post about being a co-GM - or more specifically, an Adversary player - running a huge, ancient red dragon in the excellent adventure "Into the Fire" from Dungeon Magazine #1.
I posted in the comments links to two of my own posts on the subject.
I didn't want to lose Delta's post, and I may as well connect up mine here, too, so I can find them all when I need them:
A Tale of Two Adversary Players
Lessons of Adversary Players