Have you updated to Lion (OS X 10.7)? The version of Kindle4Mac that you can download from amazon.com won't work on Lion, you have to download the (very out-of-date) Kindle app from the Mac app store. Although you'll be missing many features, you'll at least be able to read your books.
I don't know if this is an amazon bug or a Lion bug, but I hope they get it worked out soon.
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