It's interesting to guess what's the cause of these figures.
Last year, people were still used to the "if I want a muscle car, I only can get a Mustang" thinking, which explains the April 2009 figures. Then Camaros started to be seen more and more on the roads, and people got used to the idea of having choice; therefore the Mustang sales went down steadily throughout the year whilst the Camaro's were much higher.
This year May 2010, all those who were waiting for the new 5.0 Mustang bought their cars, which explain the jump; but interestingly, people still want the Camaro more even though there is no change between this year and last. This could be because of the fact that we haven't grown to miss the Mustang, given its uninterrupted history. So the Mustang is vastly improved, but most people already have their Mustang if they ever wanted one.
The funny thing is: the sales figures don't match what I see on the roads. I see far more Challengers than I do Camaros. The Camaros are stunning though when I do see one.