There's some confuddling info here. I'm not sure if some folk know what they're actually talking about.
Right.
The crucial dimension is that of the internal diameter of the headtube. This is the same on Cannondale and the onepointfive standard: 49.6mm. As has been correctly stated, the steerer diameter on Cannondale Headshock and Lefty forks is 1.56", which is obviously a tiny but significant bit bigger than the 1.5" standard.
So, you'll need a headset which fits the Cannondale/1.5 standard 49.6mm headtube. But one which is 1.5, not Cannondale fitting. They are available, as some Cannondale bikes come with 1.5 forks.
Now, older Cannondales meant for the Headshock or Lefty forks had the headtube milled to fit the relatively shallow cup of the Si headset. Some 1.5 headsets cups are too long, meaning they'd have to be cut down/the headtube milled down further.
Eg:
This is an FSA Orbit Z1.5R reducer headset, designed to fit a 1 1/8" fork in a 1.5/Cannondale headtube. I have successfully used one of these to fit a 'normal' fork to a Cannondale. No issues whatsoever. In spite of someone in a bike shop once arguing with me that it couldn't be done. Was nice to see his face when I showed him the finished result 😀
Compare with this, the FSA Orbit Exteme Pro 1.5 headset. Note how the cups are longer, ie they'd extend deeper into the frame. This is the potential issue with fitting one to a Cannondale. I've read of people cutting the cup inserts down to make it fit.
Hope this info sheds some light. Yes, you'll need a new headset. Life's not always fair.
Give Cannondale a ring/email if you're still unsure.