I'd have a look in the Ice technical Section. Technically it is a double din size faceplate. When you pull the unit out though it is installed in a Single Din cage with the Blue an Me Module (If you have it) mounted in the lower din part of the alfa specific cage. You can remove the cage and fit a standard one if you pull the center consoul out (Again in the Ice section there are walkthroughs for this) I keep thinking about repacing the standard headunit with a Satnav unit but like the fact that blue and me works well for the phone section through the wheels and don't like the amount of patch cables and the costs for getting this functionality in a seperate system.
Not impossible and lots of people seam to have done it but just have a read through first before jumping in!