well... I haven't been able to test it yet because the card I ordered is physically too large to fit in the computer without cutting some plastic, which I haven't gotten around to yet. But it doesn't work in the slot on the bottom and neither does the wireless card from under the keyboard, so it might be an incomplete implementation of the slot or it could be vendor strings in the BIOS causing us this grief. It may be possible to modify the BIOS, e.g.: http://macxylo.blogspot.com/2007/12/dv9338eu-howto-remove-mini-pci-express.html
I'd really like to try the drive in the slot under the keyboard before mucking around in the BIOS though.
Does your drive fit in the slot under the keyboard?