This should be how the skill works in normal Diablo. I would say they are not rewriting the whole engine for the x-pack so something similar should apply.All skills have a certain "length". For melee attacks one typically call it the attack speed, for spells the cast speed and so on. It can be expressed as a time in frames. There are numerous sites, calculators, lists and such that list those speeds for all kind of weapons and spells. Some skills are special in that they have a fixed length, at least some lightning skills for example used to (chain lighting?) some had a "rewind" feature (those multiple ones like Zeal and Fend(?) ).
What the game do when you perform a skill (and most things are skills, even the normal attack is a skill really is that it can do "startup work". Basically a routine that is done when the skill initiates. Then there is an update section, this is done when the skill is completed (in the mean time it just performs the animation and update your char as far as I can see). An interesting thing is that for most attacks, the actual ToHit and damage calcls and so are done upon skill initialization while the resolution is done later, this is not important here.
The WW skill will initiate and then you start out. After a certain ammount of frames (of which I really don't know as I hve not checked, it has finnished one WW "animation" and thus executes the skill resolution code. This will for WW make 2 attacks (I think the two attacks are done versus random targets within range). I leave out housekeeping stuff as target selection and other bookkeeping. It then continues with a new WW "animation" and when done, 2 more attacks and so on.
The WW stops when you have reached your target location I think although it seems you at times can be interupted.
So, how do one factor in weapon speed? Well, the simplest thing would be to apply the weapon speed modifiers (including from other items and such), just as for a normal melee attack on the "WW animation" part inbetween the attacks. As I said, I have no idea how long it is, but it seems quite short, meaing you will need some considerable modifiers for an effect but the effect will be quite large. As the game uses fixed point arithmetics one can of course do it differntly, have a higher resolution counter that you can set to a base speed of any value down to 1/256th of a frame and modify it and base your attacks on it. This means, in my opinion although I can be wrong, some basic engine changes to be done. The other alternative is something similar but with some kind of "rewind" feature that does not really rewind the counter but make sure fractions carries over (it allready might though. then nothing needs to be done).
What I am comming at is that most likely the speed effect will be the same as for other attacks, modifying a base speed by a pwercentage just lile other weapon speeds. In addition one can tweak number of attacks per attack so to speak (the 2 today).
Just some random thoughts, again, I can't check how the x-pack do it, just how the normal game do it.