See from Park Tool website
The "IS" system is considered the most common. The name Integrated System is not a registered trade name. The system uses the 45-degree bearing contact in the frame. The bearings for the 1-1/8 inch steering columns use a 41mm outside diameter with a 45-degree bearing contact in the frame. The bearing may be marked "36-45". The first number refers to the inside bearing contact with the headset race or centering cone. The second number refers to the frame contact.
The important thing for the headset bearing to fit is the outer diameter and angle. So for the "i" versions of Cane Creek headsets, they use the 41.8mm outside diameter and 45 degree angle.
However, in order to maintain compatibility with their own crown races and headset compression rings/top caps they use a 36 degree inner angle.
So in other words, it will fit perfectly, but you have to use the Cane Creek crown race and compression ring.
I went through this whole business with a Bianchi frame with Campy headset standard when I wanted to put in a 110 IS headset. According to Cane Creek customer service, even that will work (though it's the 41mm outer diameter). You just have to be sure the bearings are seated straight and properly, before tightening everything up. It worked fine, no rattling, no loosening, but I decided to go back to Campy bearings just to be safe.
In theory if you wanted to "upgrade" to higher end Cane Creek headsets (though for IS systems it's just a better cap really), you could keep the bearings and buy the ring, cap, etc for the 110 etc.
That is correct.
Just had confirmation from Cane Creek themselves... (summarised email)
The Cane Creek IS-3i is Campy/Italian compatible - that is the bearing diameter and the outside angle is same (inside does not matter - will get to that in a tick).
Campy only makes bearings that are 41.8 45/45 Hence cannondale requires this. BUT, Cane Creeks IS-3i is compatible due to the outside bearing being 45degrees and the diameter being 41.8mm (fits for a 42mm tube) The inner angle is 36degrees, hence on the fork you use the Cane Creek supplied 36 degree crown race. If you use a campy record set, then you use the campy crown race that is 45degrees.
So all will be fine, diameter and outer angle match, and will use the cane creek crown race.
Thanks for your help guys!
ps. you cannot use bearings that are like "IS-3" without the "i" as these are not campy compatible due to being 41mm in diameter not 41.8 and may come in 36/36 angles!