Root Question about rooting the Nexus S 4g

I had a friend ask me to root and update his device from 2.3.6 to 4.0.3, normally I'd be comfortable with this but he has had the carrier flashed from Sprint to Cricket so I am hesitant to do it for him ( because I only have worked with first carrier devices). I have a few questions:
1.) What are the problems that I may run into, if any?

2.) Other than nandroid back up (which I ALWAYS do) what other safeguards can I take?

3.)What warnings do you have about attempting this?