I have made a few estimates of the sales of the BlackBerry PlayBook before. Assuming that 255,000 had been sold by June 14th, and that 504,000 had been sold as of September 16th. After looking at the most recent data I calculate that RIM has sold around 700,000 tablets as of November 15th.
This shows BlackBerry tablets selling at a rate of about 100k a month recently. Sales rebounded unexpectedly in October despite the lack of a timely OS update. Numbers for November, however, are on track to be down once again.
As before this estimation is based off of the number of downloads of the free game Pixelated. While this calculation does rely on a number of assumptions, given that Pixelated has remained one of the most downloaded apps for the PlayBook, I feel that I have better numbers then most to base this estimate upon.