Greg G, you should also be able to easily get your serve velocity on your serve videos with the camera side view (perpendicular to the serve's trajectory). Most accurate if you can record a scale placed in the direction of the serve before or after the serve without moving the camera. In a pinch, use your tennis racket for a scale .
It would be more accurate to measure over a longer distance, say, several frames at 240fps. With 1/1000 sec exposure time, the ball image will have motion blur. Just estimate location of the ball by the center of the blur or best guess front edge of the ball.
Not nearly as convenient as a radar gun but it could be very accurate if done properly.
(I was going to try it with my android smartphone, Samsung Galaxy SII, but I've read that some smartphones, including iPhones, might have variable frame rates under some circumstances.)