I put together a one-page 4e Combat Crib Sheet last week for Keep on the Shadowfell and WWGD. It got some excellent critique and is now pretty polished, though I need to swap out the title/logo.
Handy to have a couple copies of it printed for 4e newbies to pass around.
I'm surprised I haven't seen more quick-reference aids myself. I have a customizable GM screen (3 panels, landscape format, front and back pockets) created for the Savage Worlds RPG, and I'd love some inserts for that. I've also seen a 4-panel portrait screen for sale somewhere. If nobody else seems to be working on it, I may have to go ahead and work up some inserts for my own use, and see if I can work out a way to share it later on.
I've made one, but it's got lots of verbatim text straight from the books (pics, even, of some of the diagrams), so I'm not sure releasing it to the public would be wise.