Mono. The video-out contact also doubles as a mic-in. Which means you can't have both at the same time. IIRC, the phone detects which is which based on impedance, but I don't know the exact details.

The Neo900 will have a stereo line-in. I don't think the details have been fleshed out yet but I assume it will reuse the earphones contacts for that purpose. If I am right, then you won't be able to monitor your line-in on your headphones but we may still have a stereo line-in and a mono mic-in at the same time.

EDIT: See here for the exact pinout.