Julian Henn has submitted a playable classic style shoot-em-up:

This is my first game I've written for the GBA - a simple shoot-em-up game,
written in Catapult. The graphics are fairly basic but the game's quite playable, and features high score saving.

This has some nice touches like acceleration for the controls of the main ship and bouncing bullets that can hit enemies from behind
http://www.gbadev.org/