I can't take credit for this, as an original idea... I noticed a few other guys had done it, and thought it looked fantastic, so I decided to try it myself. I'm happy with the results.
On a scale of 1 - 10 for difficulty, I'd rate this as a 2 (easy).
First, I painted my calipers black, using a Duplicolor Kit ($25).
Then I ordered some decals from this guy, on E-Bay. He ships from Italy, but they arrived in 4 days! (he's got a few different styles)
The decals were quite easy to apply. I messed up the first one (I'd recommend ordering a couple sets). After applying the decal, I sprayed over the caliper with clear coat, to seal it.
It's subtle with the wheel on... but still looks cool!