If you google on "kindle development kit", you'll get most of the known info. Basically the kdk was released as a closed beta (along with a "special" 2.4 kindle firmware upgrade) back in February. Some documentation on the kdk has been published -- as has some info on app selling and compensation rules.
Some have speculated there will be a kindle app store; others have surmised it would be folded into the kindle store. Some speculation (sorry can't find link) about a fall 2010 release.
There's been a lot of ferment in the kindle space in the past few weeks. My personal guess is that there's still more to come. As the OP noted it's been very quiet -- given the changes to the competitive landscape, and Bezos' recent comments targeting kindles at "serious" readers, they may decide to back off the app idea
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