Okay, i used the documentation on DSiBrew, plus some staring at the PPM files in a HEX editor, and have come up with this:
http://www.mediafire.com/?clt4ad2wcmhIt is a CMD app that is able to extract frames and the preview image from a PPM file and show the header info!

The PPM file is the file containing the animation of a Flipnote(DSi only) Not the image format supported by GIMP
Edit: Updated to a newer version