D70 sample, Why so dark?

Welcome to the great mystery and frustration of flash photography!

Next time, put your camera in Manual mode 1/60th of a sec f/5.6, if your flash is ETTL you should get brighter results the slower you set the shutter speed. aka, if you shoot 1/50 5.6, the background will be brighter than 1/60, and 1/40 will be brighter than both of them.

I shoot the D100, and find Nikon's flash system to be a lot less sophisticated than Canon's, most of the time I find myself shifting flash exposure down 1.3 stops on the Nikon, while with the Canon's is more shoot and go.