Around here there is a saying that crows can count to 2. So as the story goes: If you are going to hunt crows from a shed you should walk 4 men into the shed and then 3 should leave. Then the crow will count one, two, many men went in and many men went out and will consider the shed empty. I don't know if it is real but I have heard it from farmers/ around where I grew up.
I wouldn't call some crows falling for a trap a clear indication of not being intelligent as there are probably many that also avoid it. Even humans fall for obvious traps again and again that many of us also avoid (phishing, Nigerian scams, pyramid schemes, link jacking, etc).
Not trying to be a defender of animal intelligence, just playing devil advocate for the sake of good, intelligent conversation :)
Then again: Crows aren't too intelligent, the ladder trap seems to be effective around the world - http://www.youtube.com/watch?v=LZLYiOn83Po