As a note, you can use a program like shred on a Linux desktop on the MMC device that the N8x0/N900 exports over USB (when it's unmounted), but as mentioned, that probably won't be terribly effective with flash memory. If you can't think of anything else, then you can always try it, but don't rely on it. Also be aware that if you're using a journalled file system on the card, it's usually a moot point anyhow.