I stumbled over a great site called eReader IQ, that does a couple of the things I wish Amazon had done themselves. The two best features are a) it lets you enter a link to a Kindle book so that you will be alerted if the price goes down by at least the amount you specify, and b) you can also enter a link to a print-only book, and it will alert you if a Kindle version becomes available. Some other useful features include a page of free Kindle books, and an advanced Amazon search interface.
It's a cool site, and since it's supported by advertising, it's free! If you ask me, publishers should support this site, because it does what they can't do: it gets around the fact that people don't wander into Amazon and browse the virtual shelves, or study the new ebook release table. Having a way to alert potential customers that a books is available and/or is now in their price range is a great thing for publishers. And if they supported it, maybe the site owners would add Barnes and Noble and iBooks support as well as Amazon?
Let me know what you think if you check it out. I thought it was way cool.