Many cameras have a special autofocus sensor that becomes active when the maximum f-number of the lens is f/2.8 or faster. It has better performance than the normal autofocus sensors because it has a wider baseline, as illustrated in this diagram from the Canon 1D3 manual:



This PDF has a great description of how it works:

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

Sometimes it is said that f/2.8 lenses are better for AF because they allow more light for the AF system; actually, the reason is that it allows the the wider baseline of the f/2.8 AF sensors. Also, there is no improvement in going from f/2.8 to f/1.2.