This is really a factor of understanding Ohms Law and the power curve of the alternator.

If all parts of two identical turbines were designed properly (12v and 24v), there should be minimal difference between the two at what RPM they stalled. The stalling usually takes place when you attempt to load the alternator before the prop gets up to an acceptable RPM. An easy way to avoid this is to make sure you wind the stator for cut in at a reasonable RPM. If you shoot too low, you will stall the prop and prevent the machine from working optimally in higher wind speeds. Sometimes this is acceptable because in most battery charging systems, the bank will be charged early on and some of the power from the higher winds can be wasted by allowing the machine to furl,...in other words, it makes for a slow, peaceful machine. This is not optimal for grid tie systems however. A compromise must be made, and in most cases, MPPT will help out big time by widening your operating "window".

With regard to the voltage difference and stall, this has to do with the resistance of the electrical circuit running up and down your tower. The circuit is comprised of the stator, the rectifier, and the feed line coming down the tower. The feed line and the stator must be thought of as one piece. If the resistance calculations done when building a stator, took the feed line resistance into account, I think it would be a little better understood. A 12v machine will have 2x the current as a 24v machine. This means that a larger feed line will need to be used to avoid line loss. If you use too large a feed line, you run the risk of stalling. Too small a feed line, and you run the risk of over speeding in high winds. It is important to have the right amount of "coupling" to have a workable system, and still attain stator longevity. I believe the latest trend is to use larger magnets than normal. This will allow you to have a tight coupled rotor/stator and tends to make the stator less prone to burn out from high winds because you can use less copper and keep your resistance lower (furling is still very important). The resistance is then added to the feed line to make up the difference.

All things being equal the 12v machine would be the least likely to stall as the line resistance has greater effect and so does the diode drop.

I suspect that many people pick up winding details for higher voltage machines and try to use them at 12v. There is a mistaken belief that using more turns gives better results and it doesn't and can really spoil the performance.

As you say, stall is related to blade speed and angle and the system voltage doesn't come into this directly.

The best you can hope for is to choose a cut in speed where the blades are rotating above the ideal speed but still near enough to give some results in the lightest wind.

The tsr falls very rapidly with load and you will soon come on to the peak of the power curve. As you load more you fall below optimum and if you get things right you should just be approaching stall at the time you need to furl to protect the alternator.

If you cut in too early you start on or below the peak of the blades curve and you run into stall within the working wind speed range. Once you hit stall badly the power remains virtually constant with increase speed.

http://www.thebackshed.com/windmill/articles/DonBrown1.asp

Those certainly are the main causes of stall in windmills.  I don't mean to complicate this any more, but it seems to me that some , at times, also refer to the aeronautical aspect of the blades themselves "stalling" in low rpm rotation because the airfoil shape (curved backside)of the blades cannot get to the point of acheiving 'Lift', because of being 'held back' from picking up enough rpm from the reasons just described from everyone's answers.  

This might be really saying the same thing.  I am still learning myself on this, but is it possible, in a rare occurance, that stall could also be caused by the blades themselves, apart from the stator or line resistance??
Perhaps someone who knows could comment?

I believe I understand everything exept this one statement which others have said in the past also.  But the blades are FIXED at ONE ANGLE (say 4 or 5 degrees at tip) to the hub, so how can the 'angle of attack' ever change!?  The blades don't twist.  I always figured it was related to SPEED of the blades moving thru the air in rotation the way the air either 'falls off' the back side of the blades or 'wraps' around the back side of the blades??.