I ordered a set from Bimmian, paint-matched to my Zed's body color. Silly, perhaps, as just about any off-the shelf black plate frame would have matched Jet Black well enough. But what made it worth it is that the style of Bimmian's plate frames visually "shrinks" the plate fairly well. A little pricey, but worth it.
http://www.bimmian.com/Z4/PPF/
I've also seen plate frames that disappear under the car, either manually or motorized. It's kinda James Bond-ish, but a bit silly all he same.