Can I recover a deleted video from the iPhone 4S camera roll?
If it is jailbroken and you have OpenSSH installed, you might be able to pipe a whole disk image to your computer, on which you will be able to try standard HFS+ file recovery software.

If you don't meet the above requirements, keep in mind all disk activity - especially that involved in the jailbreaking process, but even that which could have happened in the meantime by normal use, has the very potential of contributing to the overwriting of the affected files.