I cannot believe Apple can have such a common and show-stopper bug. It did happened.
After I upgrade my iPhone to 4.3.3, all my downloaded application (from App Store) crashes. I can only use the native applications.
The solution was also amazingly simple:
Delete any App, and download it again from App Store again. Restart that App.
Not only that App works, all the app works normally. If you want more certainty about the steps, I delete Instagram and reinstalled it. Then it worked.
Hope this helps.