Wasn't the iPad jailbroken in a day? The code might not have been released the same day, but you best be sure they were hard at work on it the moment they got it.
However, the iPhone 4 may be a different beast altogether. While it has a different OS, it will likely have a different baseband and bootloader. Maybe Apple will have plugged up all the known exploits by now... You do recall they were hiring a security expert a while back.