After updating to iOS5, Safari will not run on jailbroken iPhone 4
It's because I jail broke the device with a tethered jailbreak. A tethered jailbreak requires you to use the jailbreak software (viz., Redsn0w) to boot the device (the option "just boot tethered") whenever it is turned off (hence "tethered").

If I don't boot the phone with the jailbreak software, Safari doesn't work (along with some other default applications). All my fault.