What is the (jailbroken) app for iPhone that allows you to replace the at&t at the top with your own words called? Will it work with iPod touch?

You can do this. The App is called Make it Mine and you can download it here.

Remember to remove this when you restore your iPhone or iPod Touch, or when you take it into an Apple store or service center.

I found that makeitmine doesnt work. Try “Status style”

makeitmine worked for me.