In Diablo 2, attack rate was calculated in terms of frames. This meant that there were discrete jumps as you broke certain Increase-Attack-Speed breakpoints. In Diablo 3, is there a similar discrete frame-based calculation, and what is the framerate used by the game engine in calculating attacks?
This is an old question, but all the literature suggests that Diablo 3 has a 60 fps simulation rate, and that IAS should be tailored to hit an integral number of frames per attack at 60 fps.
Diablo III is not sprite-based like Diablo II was (Instead, it's full 3d), so there are no longer discrete frames.
The gameplay ramifications of this, of course, is that any amount of increased attack speed is equally viable. (Though not necessarily noticeable)
You can view your current attacks per second on your character sheet, and unlike in Diablo II (which listed only a speed, e.g. Slow / Fast / Very Fast), you'll actually get a decimal value.