I get asked quite often what media formats a BlackBerry supports. While some of them are obvious like MP3 files others are not. For example, do you know if BlackBerrys support the enthusiast FLAC or OGG audio codecs? (teaser :))
RIM happens to publish a PDF document titled: “Supported Media Types on BlackBerry Smartphones” (direct PDF link) that is the perfect guide to find what media files your BlackBerry supports. If your media file does not play natively on your BlackBerry they also offer some pretty nice instructions on how to get it to work by converting/transcoding the file. (Using some opensource software :))
Let us know if you still have any questions. Personally I love the XVID support on my 9800.