There is a photon "covert" model with a shrouded LED, but they have multiple modes so I guess it isn't what you're looking for.

Coast, LED Lenser, and Eddie Bauer are US distributors of Zweibruder flashlights. Short of the photons, these are the best I've seen.

They have several keychain models, the thinnest of which I use in my PSK because it's so out of the way. This is a combination twist/push button tail type so it's out of the question.

Two of the slightly bigger keychain model, the V8 and V9, has the pushbutton feature you're looking for. I have the blue model V8, and it's bright enough that if you shine it at the ceiling in a dark room, it bathes your entire room in a blue glow enough to get around in. Push the button and you have momentary light; push harder to turn on hands free. It has a clip on it that can be positioned over the button to protect it, and it's flexible enough that you can push momentary on.