Does jailbreaking mess up anything with AT&T or Apple?

Nope unless you don't know what you are doing. My first gen iphone has been jailbroke since the first day I've bought and now I am using 3g and jailbroken. Crash my phone couple of times when I was trying to SSH and messed up some of the system files but always restore back and re jailbreak again. No problem at all. Since you can restore back, you won't have any issue with AT&T or Apple. Just restore it back when you need to.

I've done unspeakable things to my iPhone, and I've always been able to recover.

all you do is install with whatever the current "tested" version supports. the devteam will tell you what 100% works.

there are lots of different apps in cydia, and they ALL have in depth desciptions. if you are unsure of something that you dont understand then DO NOT INSTALL IT.

another general rule i use is:

if i dont know what it is, and it may mess up my phone, i do not ever install it when i dont have an hour to fix my phone immediately.
so no installing strange stuff when out, and not before a big trip, nite out to the movies etc.

makes sense right?