If you take their data at face value, IE plateaued in late 2009, early 2010 after a long downward trend (from nearly 60% down to about 45%) in Europe. IE’s share only started to go down again in April and from then on slowly but steadily dropped to its current low level. That could support your hypothesis. The browser ballot came with a Windows update that was delivered on March 17 2010.

The only problem I see is that the current downward trend is nothing special if you zoom out a bit. As I already said, that plateau was preceded by a long downward trend. 2008 and 2009 look just like 2010. (StatCounter has no data from before July 2008.) IE has been steadily losing browser share for a long time.

I’m consequently not ready to make any definitive statements about the effects of the browser ballot.