Wikipedia has some interesting information on this subject.
http://en.wikipedia.org/wiki/Exercise_(options)
In particular, note the paragraph under "Early Exercise Strategy".
"A common strategy among professional option traders is to sell large quantities of in-the-money calls just prior to an ex-dividend date. Quite often, non-professional option traders may not understand the benefit of exercising a call option early, and therefore may unintentionally forego the value of the dividend. The professional trader may only be 'assigned' on a portion of the calls, and therefore profits by receiving a dividend on the stock used to hedge the calls that are not exercised."
http://en.wikipedia.org/wiki/Exercise_(options)
In particular, note the paragraph under "Early Exercise Strategy".
"A common strategy among professional option traders is to sell large quantities of in-the-money calls just prior to an ex-dividend date. Quite often, non-professional option traders may not understand the benefit of exercising a call option early, and therefore may unintentionally forego the value of the dividend. The professional trader may only be 'assigned' on a portion of the calls, and therefore profits by receiving a dividend on the stock used to hedge the calls that are not exercised."