Any lawyers here?
I understood no contract can work to waive people's rights, such as this clause mentioned in the post above " AT&T's subscriber contract prohibits class action or jury trials"
You can absolutely waive rights. Unless there is clear duress, the clause is unconscionable or against public policy (or something like that). In cases like this, you can always NOT sign up. Nothing is forcing you to get an AT&T iPhone. Or any phone for that matter. They are not FORCING you to sign up for their service.
Contracts are drafted everyday that require the parties to first arbitrate or mediate or disclaim all kinds of things. As long as there is valid consideration (a legal term that really sucks in law school) its all good.