Amazon just announced it today. From the press release:
Amazon.com, Inc. (NASDAQ: AMZN) today announced "Kindle for Mac," a free application that lets readers around the world enjoy Kindle books on their Mac computers. The U.S. Kindle Store (www.amazon.com/kindlestore) currently offers over 450,000 books, including New Releases and 102 of 111 New York Times Bestsellers, and is the only place to find some of today's most popular books in digital format. Kindle books can now be read on the Kindle, Kindle DX, iPhone, iPod touch, BlackBerry, PC and Mac, and soon the iPad. Customers in over 100 countries can go to www.amazon.com/kindleformac to download the Kindle for Mac application for free today.
Kindle for Mac features Amazon's Whispersync technology that automatically saves and synchronizes bookmarks and last page read across devices. Whether customers are reading Kindle books on a Kindle, Kindle DX, or one of the free Kindle applications, they can always have their reading with them and never lose their place. With Kindle for Mac, a customer can read some on their Mac, read some on their Kindle, and always pick up where they left off.
As software goes, it's really not much. I wish Amazon had continued to develop the Mobipocket apps; they could do so much more than Kindle.