This is a pretty nice, stealth boost gauge option by a small third-party vendor. The Boost PSI gauge uses orange lights integrated fully into the driver's left side air vent.
There is a new Do-It-Yourself thread on Bimmerfest that shows how to install the product. Seems pretty straight forward and looks great in the accompanying videos in the DIY Thread. more.
UPDATE: The product has recently been updated with a slightly tweaked front face to better match the interior's matte black finish. More info here: http://www.rixeffects.com/p3/ventgauge.