[Kracka]

How long do you suspect that cooling fan has been non-functional? Are there any signs of leaks now that things have cooled off? If the impact in some way damaged the radiator fan, and that allowed temps to creep up, that may have been just enough to expose the cap's weakness. If you cracked the radiator, there would be a puddle under the car (besides the one underneath the overflow tank).

As far as you can tell, did anything else make contact with the curb besides the wheel/tire?