With a flash that has TTL control you can just set a long exposure time and let the camera turn off the flash when the foreground is sufficiently exposed. The shutter will be open for the specified time and let in the light from the background. I don't know if you can use this method with your camera.
Left: 1/60 s. f/4.0
Right: 1.6 s. f/5.0
alt text http://www.guffa.com/Temp/flash.jpg
