Originally Posted by wildjim
It would seem that a frontal impact with a curb hard enough to damage the frame would also damage the front wheel?. . .
This is not true at all. Intuition may cause you to think this, but it is not the case. Wheels are incredibly strong; they posess a strength to weight ratio of 400 to 1, which is among the highest for any man-made structure. I could ram all my bikes into a parked car and destroy every frame and fork and have perfectly good front wheels.
Bike shops and manufacturers get this situation somewhat regularly. It's called JRA. "I was Just Riding Along, when...." Be sure not to take this approach. It won't work. The previous posts describe how your frame failed: the force was transmitted through your front wheel to your fork, which acted like a lever arm on the head tube. This type of failure is not that uncommon. If bicycle manufacturers designed bikes to withstand high force frontal impacts (and they easily could), they wouldn't sell very many. In other words, the bikes would be so heavy you wouldn't want to ride them.
So, manufacturers try to find a middle ground. The bike should be able to take a certain amount of frontal force, while maintaining low frame and overall bike weight. Some manufacturers purposely design their frames and forks so that the fork will fail first in the "frontal impact" situation. But guess what? Customers complain that their forks failed! You've saved them from a frame failure, but the consumer only sees a failed fork and are thus unhappy. It's somewhat of a no win situation.
In your case, if you really were going 3-5 miles per hour under the circumstances you describe, then perhaps you have a case. However, you mentioned that the top tube was deformed as well. It takes a great amount of force to both buckle the downtube and bend the top tube as well. If the downtube failed and the top tube were not bent, buckled or deformed in any way, then perhaps there was a problem with the downtube. But this isn't the case so this leads me to believe there was more force involved than what you described.
Either way, approach your LBS with a solution-oriented approach. Leave the accusations and defensiveness at home. Most manufacturers offer a low-cost frame replacement option for cases like this. And there's always the off chance that there may have been some manufacturing defect that's only now coming to light on your model.
Good luck!