Klegg Electronics, Inc. announces the Klegg mini mp3 player and claims it is the world's smallest mp3 player with a color display. Let's have a look:

The Klegg mini measures 1.8x1.6x0.5 inch (45.72x40.64x12.7mm) this results in a volume of 23.60cm3. An Apple iPod nano (3.5 x 1.6 x 0.27 inch or 88.9x40.64x6.8mm) has a volume of 24.56cm3. So the iPod nano is a bit bigger in volume than the Klegg mini.
The Eratech EMP-Z2 egg-shaped MP3 player is also bigger than the Klegg mini. Unless I missed an MP3 player with color display, then the Klegg mini is really the world's smallest of that kind. And hey, it comes in pink. Features include 128x128px screen with 4098 colors, photo viewer, mp3 and wma music file support, voice recorder, USB 2.0 The Klegg mini comes in 128MB, 256MB and 512MB. The Klegg mini starts at $79.95

More details on the Klegg Electronics site.

The MobiBlu DAH-1500i has only a single color OLED display, but it is way smaller (24x24x24mm - 13.8cm3).