I was forwarded a copy of this Cannondale USA official response, the first two parts are excerpts from the warranty info.
Cannondale wrote:FRAMES: (frame, fork structure, swing arm): Cannondale frames (except frames for Freeride, and Dirt Jumping bikes, see below) are warranted by Cycling Sports Group, Inc., 16 Trowbridge Drive, Bethel, CT 06801 against manufacturing defects in materials and/or workmanship for the lifetime of the original owner.
Damage resulting from normal wear and tear, including the results of fatigue, is not covered. Fatigue damage is a symptom of the frame being worn out through normal use. It is one kind of normal wear and tear, and it is the owner’s responsibility to inspect his/her bicycle. See Section 5. and D.
We consider the case of your frame being part of the normal wear and tear (fatigue). Your frame is therefore not covered under warranty.
The gentleman affected (which is also covered in more detail in this thread on Cannondale Warranty) asks, rightly so, "So what do they cover?". I would like to say that I am surprised at the response by Cannondale, but this is not the first time I have heard this. Selling Cannondales with a Lifetime warranty is misleading if damage as a result of normal use is not covered. A 5 year warranty Realistic and acceptable.
When a buyer purchases a bike with a lifetime guarantee they are highly unlikely to read the terms and conditions or warranty information before purchase - in store a buyer accepts that the frame has a lifetime warranty which is generally understood to include normal usage. To demonstrate the point, Cannondale can state that every breakage is a result of fatigue and normal wear and tear - so the only situation that you could expect (or hope) that it was covered under the lifetime warranty is if it was never riden.