Last year the PRS-300 and PRS-600 leaked on 07-30-2009 (
New Sony model to be called PRS-300 and PRS-600) and were announced on 08-05-2009 (
Official Sony PR re PRS-300/600). In previous years, the PRS-505 and PRS-500 were delivered at about the end of September.
So I would expect something in the next two months, and it would make sense to pre-anounce before the Kindle WiFi models start shipping. However, the recent Kindle/Nook/Kobo price wars must raise a serious question at Sony about whether they can compete. Sony is strong outside the US, but I don't see how prices can be maintained worldwide with Amazon shipping to so many countries.
Note that you can order a Kindle and cancel before delivery (although the case may ship first), and even return it within 30 days.