Good chance there will be an engine change in 2011, but that is not the year to buy. I would never buy a new engine (in this car) the first year. Let someone else be the test driver.
The Shelby contract was renewed for years 2010, 2011, and 2012 but is not expected to be renewed beyond 2012.
So based on the above information I plan to buy one of the 2012 GT500s as it will be the last year of the Shelby GT500 run and will have a proven engine/car/drive train installation.
I plan to keep the 2010 I have purchased as it will also be a limited run with this body style and engine.
I would also look for a price increase in 2011 and 2012.