Combining sharpness and blur in the same photo is a great way to imply motion. When Joe McNally was on assignment shooting dancers in Dubai, he did it twice in this awesome photograph.
He uses a Nikon D3, and in this case he set it for a triple exposure and bookended the flashes. He explains how he did it (a little) in his post here.
I'm also working my way though his excellent book, The Hot Shoe Diaries. You can get it at Amazon.