Microcenter was still selling new 2009 MacBook Pros in August and September 2010. They had great discounts on the 17" C2D for about $700 off for a brand new, one year old model.
I would assume that some retailers and online stores will have the old stock for quite awhile but Apple Retail will get rid of them fast.
Last month I saw MacMall selling a brand new 2009 Mac Pro 2.93 Ghz Quad-Core, 1 TB HD, 6GB Ram, and high-end ATI Graphics card for $2049.
The same day, Apple Online Store was selling the same computer 2009 2.93Ghz Quad-Core refurbed, with 1/2 the Ram (3GB), 640GB HD, and crappy video card for $2749.
In the past, people could usually get a 10% discount on the current/old model the day the brand new model is released.
My guess is that you will get a much better discount on last years models at a 3rd party retailer.