The MR9800 card HAS to work with the i9100 (even the older versions). The XPS models can get the card as an upgrade, and the i9100 and XPS share the same motherboard. So either Dell is screwing over the XPS owners from getting their promised upgrade, or the techies gave you a BS line about not being compatible (I'm thinking you got a BS line from the techs).
I won't fret it though. I trust that eventually we'll be able to order the part. Just gotta wait it out.