I jailbroke my iPhone 3G with firmware 3.1.3 and after doing so, any new apps I installed would crash when trying to open. It was frustrating to see everything close and be back on the home screen. I finally fixed it:
In iTunes, select the iPhone under the list of devices.
Go to the Applications tab and click "Sync Applications."
Once the Sync is complete, the apps should open. Very odd bug, indeed.