Wickeycolumbus released 256 Byte Pong for Atari 2600.

Release notes:

After compleating Pong 512 Bytes, I decided I would attempt to cut the ROM size in half to a measly 256 Bytes. It is not down to 256 bytes yet, but it is getting there at only 343 Bytes. Some features had to be cut to slim it down like scoring and sound. This version was tested (and works) on real hardware. Here is the latest binary/source code.

http://www.atariage.com/forums/index.php?showtopic=139216