Straight from Nintendo.com
"Can the DS play Game Boy games?
What it can play:
* Game Boy Advance games in single-player mode only
* Game Boy Advance videos
What it cannot play:
* Original Game Boy or Game Boy Color games
* Game Boy Advance games in multi-player mode
"
It looks slightly better on the DS too. Had some Fairly Odd parent videos, and they looked fine.
GBA Video wont work on the GBA Player though.