Some clarification - Cannondale's Headshok predates the 1.5in standard, but the internal diameter of a Cannondale Headshok headtube and a 1.5in frame headtube are the same. The only difference as far as the frames are concerned is that Headshok headsets don't go into the frame as far as 1.5in ones. That means that you can put a Headshok headset into a 1.5in frame, but you may not be able to put a 1.5in headset into a Headshok frame - some of them had steps inside that would stop the headset going in all the way. Original Headshok steerer tubes were 1.565in diameter, so a tiny bit bigger than 1.5in. I'm pretty sure that the current ones are 1.5in, though.
Hm. That's probably not really clarified anything at all, has it? ;-)