You're probably thinking about Magura's replacement cartridge. Mountain Bike Action had a brief review maybe a year or so ago and it seemed favorable. Only caveat is that the top of the unit stick up about 2 inches above the top of the steerer (the black lever and red dial).

http://www.magurausa.com/mountainbik...cannondale.php

At US$275 it's a bit out of my price range, but it sounds cool enough.

Hang on-- you say you CANNOT lock out your suspension? I've never heard of a headshok that can't be locked out. Are you sure that your suspension isn't broken?

As far as i know, the most basic adjustment on Headshoks is lock-out with other adjustments (damping, rebound) being present on more expensive models.