Heck, all that is is a very bright LED display panel hooked up to a speed sensor(the display is reversed so that when it reflects on the inside of the windshield it's right side up and reads left to right). If you get a digital tach or digital speedo, you could build your own "HUD" to hook into it for about $20 in parts for the display.
That is a good idea. I was just thinking that this couldn't be too hard to replicate. The only tricky part would be making it actually look cool. People would always be asking: "What is that?". Might try to find a way to hide it in the defrost vents.
Yeah, hiding the display so it didn't show from the outside would be the trickiest part. A simple shade blocking the front of the unit wouldn't work because it would still have to be shining up at the right angle to reflect off of the windshield so the driver could see it.
Perhaps some technique using polarized film would do the trick. Another neat thing for daylight would be a TFT LCD film that stuck on the windshield similar to those see-through clocks.
I'm sure southpawboston would have some ideas. I'm not all that electronics literate, but I think what you would need would be a digital speedometer unit, then either a very bright LED display or the see-through LED display.
Usually, each segment wires to a contact near the edge of the display, so if you wanted it to be reversed, just wire it reversed. I haven't seen any LCD displays on my normal overstock electronics site but they do have three digit LED displays (http://www.excess-solutions.com/displays2.htm#3-Digit_Displays). I don't know how much trouble it would be to get into the stock display and connect to the existing wiring, though. It might take a very fine touch with a soldering iron to do it right.