I am writing this post on my iPhone using the Bluetooth keyboard I bought at Brookstone over the weekend. It is made of silicone and it rolls or folds up, it's lightweight, and it has a built in Lithium Ion battery. So it has all the features I wanted.

On the downside, it is easy to miss a key or skip a letter if I don't type carefully. But I'm finding this improves with practice, so even though it will take some getting used to, I think I might keep it.

UPDATE: I have gone ahead and preordered a different keyboard: the Jorno.  It is not available yet, and it's supposed to ship March 31st.  I think it'll be more durable for traveling, and it is probably closer to a full-size keyboard, but it weighs just over 8 ounces and has the built-in Lithium Ion battery I wanted.