To add to what rfusca stated in his answer, you should look into using second-curtain flash. Flash is also a powerful action-stopping tool, but is often overlooked. When using flash, the pulse is only a tiny fraction of a second long, and usually occurs when the front curtain of the shutter opens.
To capture motion blur, but also freeze the subject's motion, second curtain flash sync can be used. Rather than firing the flash pulse when the shutter initially opens, using second-curtain sync will fire the flash pulse a moment before the shutter closes. When combined with "slower" shutter speeds, such as 1/30th of a second (or slower...depends on how much motion blur you want), the flash will freeze your subject clearly at the end of the exposure time.
Second certain flash sync is used to great effect in areas like sports. Most of us have seen those fantastic shots of a baseball player running for base, with a beautiful trail of motion blur behind them. Or of a basketball player flying in for the dunk, motion blur showing their trajectory. An excellent example of this is this cyclist racing around a corner (found via Bing...no reference site):
I would give second curtain flash sync a try. If you are close to your subjects, you will probably want to dial down the flash power (flash exposure compensation). You don't really want the primary lighting to be from the flash...you just want to use it to freeze your subject.
Here are some resources that might help you out: