That's right, a 2007 MacBook Pro prototype has surfaced on eBay, and it comes complete with "a 3G antenna, 3G hardware and SIM card slot built in." Unlike standard models, this one "carries a red motherboard which is typical of Apple prototypes." Click here for more pictures.
The seller also notes that the remnants of the SIM-card connectivity remain on the production motherboards but are not populated.[via MacRumors - eBay]