Scooper

Yep, center of steerer tube opening to center of handlebar clamp. To avoid confusion, though, this stem dimension is usually referred to as the stem extension, not the stem length. Here's a quote from the Rivendell website:

"Stem length numbers below refer to the FORWARD EXTENSION, MEASURED CENTER TO CENTER ALONG THE SIDE FROM THE CENTER OF THE QUILL TO THE CENTER OF THE BAR CLAMP, not the quill length, which is a constant for a particular model of stem."

Nitto Technomic Deluxe stems, for example, are 190 mm long (quill length) with extensions from 70 mm to 120 mm.