A little late in the reply, but one option is to take a longer exposure with 2nd curtain sync on the flash.
Meaning, you can take like a 1 second exposure so that you capture all the ambient light and mood, then just as the 2nd curtain is about to close (at the end of the 1 second exposure), your flash will fire and fully illuminate your subject.
Here, check out these photos at flickr tagged "2nd curtain"...
http://www.flickr.com/photos/tags/2n...n/interesting/