Well IMO, the valentine 1 is the best radar detector available.
http://www.valentine1.com
While you do have to mount this on your windshield, it DOES do front, rear, side to side detection... AND it has arrows on it to tell you what direction the cops are coming from. It does X, K, KA, and Laser with POP detection.
Also, sorry to kill your idea about the rear view mirror, but someone already beat you to it.
http://www.ecisbmw.com/product_files...%20Display.htm
Pretty neato deal... there are some companies out there that will actually take your mirror and do this upgrade.... I personally think it is a little pricey for what it is, so I just go with the regular unit on the windshield...
BTW, I went from a Uniden detector to the V1 about 4 years ago. At that point I was 1 ticket away from losing my drivers license do to all my excessive speeding tickets. Since the V1 purchase I have had only 1 speeding ticket, the V1 was beeping like crazy, and I just was ignoring it... thinking that it was a false alarm since I was out on a country road and nothing was around... Well the Fing cop was out in a farmers pasture hiding behind big rolls of HAY! UGH!!?!
Hope this helps.