Today I bought a new microSD card for my phone. My son also wanted a SD card for his Nintendo DSi. He wanted to use it to put music on for playback. So I figured that I would give him the 2GB card that was I was using in my phone.
When I copied some mp3 files to an SD card, and they were not playable. It looks like the Nintendo DSi only plays AAC files. On their web page they link to the Nero software where they sell it for $6. You can do the same thing for free with iTunes.