I haven't implemented pong, but I've got video output working for NTSC. What I've done so far is easier than pong because there are none of the branches in code that have to all meet timing, but I'll play with that later.
Code:
http://binarymillenium.googlecode.com/svn/trunk/arduino/videontsc/video5from3.pde
Video, with incorrect timing demonstrated:
Writeup:
http://code.google.com/p/binarymillenium/wiki/ArduinoVideo