You can swing a KB low (belt level) or high (head level) or overhead (as seen in the picture below). The two important components are the complete hip snap and the circular arc with straight arms.
Below is a picture found at
www.geocities.com/Colesseum/Field/7342/Warmup.jpg
It's a picture of the Great Alexeyev warming up with a 32kg KB, swinging it overhead. This pic was taken around 1970-71, before many CFers were born.
I post this to show that there is nothing wrong with doing swings head high or overhead or even low. They all have different effects and usages. For beginners (and for others moving up to a heavier KB) it's best to start doing your swings low. As you get stronger/fitter, your swings get higher and higher. The safety factor is that you don't want to generate more force than you can handle on the back swing.
Another thing to consider about doing overhead swings is that you have to squeeze the handle twice (at the top and the bottom) will doing them, as opposed to only having to squeeze it once while doing chest high swings. Your grip can become the limiting factor with heavier bells. Being able to do twice as many reps with a heavier bell should be evaluated. See if doing 50 chest high swings with a 32kg KB is as taxing as doing 25-30 overhead swings with a 24kg KB.
Another suggestion for CFers that like to do overhead swings, be sure to squeeze your abs and butt at the top, to protect your back and to prevent arching of the low back.
p.s. Yes, I'm the (in)famous "T.C." mentioned in the CF Journal on the American swing VS. the Russian swing. What I WISHED the Journal would have contained was a primer on how to learn how to swing safely and powerfully, no matter how high you go. Overhead swings are an awesome drill, but done in a herky-jerky style, with slightly bent legs and an arched back is a recipe for disaster. This can be seen on page 7 of CF 25. The best swing technique shown is on on page 5 at the bottom. Legs straight and locked, hips completely extended, low back nuetral, glutes flexed, arms perfectly straight with the bell in line with them. Excellent form.
http://www.crossfit.com/discus/messages/22/13739.jpg