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The one I am waiting on with baited breath is the TouchFire keyboard. I'm not certain when it will be released, but it should be available soon. The design is brilliant in its simplicity, essentially a membrane overlay that replicates the shape of keys while using the iPad's touch screen keyboard, so it would definitely meet your desire for light weight and thinness. How it feels will be interesting to experience, since it won't have individual keys per se. Still, having a tactile point of reference for touchpad typing will be a massive improvement even if it isn't like one of those nice 80s vintage IBM keyboards (mmmmm, typing nirvana).
Of the choices available now, I like the Logitech by ZAGG iPad2 case/keyboard/stand which costs about $100. One benefit that it provides over a pure keyboard is serious drop protection, something that can come in handy with such an expensive portable device. It adds some thickness and weight, but not too much, and it still lets you access your screen while you look all cool and techie :-)