The problem isn't just the height. Shank is right that you can cut the dash plate to a bigger (double din) height, but the opening in the dash is already wider than aftermarket head units, so there will still be a gap around the sides of the head unit that will have to be filled in with a bezel or other custom plate.
I'm still trying to decide what to do to get a double din in my dash, also. I don't like the thought up of the flip-ups, since they cover the vents above the head unit. And I'm too fussy to live with a raised bezel.
I figure I could buy a replacement dash, cut the radio opening to double din height, fill in the sides with poly-resin and grind smooth. The hard part will be finding a way to resurface the entire dash afterwards. But I have a commemorative edition with the faux-brushed aluminum finish, so I might have en easier time of it than others.
Unless of course, someone starts making dashes with perfect double-din cutouts beforehand, then all I'll need is my wallet.