If Amazon supported epub I would very likely buy a Kindle. The lack of epub support is pretty much the one reason why I didn't go for a Kindle in the first place; they have better features for less money than pretty much anything else on the market. I would start buying ebooks from Amazon at a last resort if I couldn't find them from my preferred retailers; unfortunately that's quite often the case because Amazon has a better selection than pretty much anyone else.

However it seems to me like this breaks Amazon's business model, which is to sell Kindles at a loss so that people get into the habit of buying ebooks from Amazon directly from their devices. If it were easy to get free content for Kindles, or get books from other retailers without jumping through extra hoops, where's Amazon's market advantage? I suspect that they'll support epub like they "support" mobi: with their own proprietary DRM so that you're still completely locked in to Amazon. So they won't be getting my extra several hundred dollars per year after all.