This is common information. It gets rehashed every time an iPhone comes out.
Here's the deal, guys. If you're an AT&T customer already, you can't do any manipulation of lines, etc without being subject to the 90-day restriction. If you call and cancel one line, you will not be eligible to add another line for 90 days. If you cancel the whole contract, you can't get another one for, yep, 90 days.
This means you WILL lose your number, since numbers reenter the pool after 30 days, and you can't request a new number when you go to get the new phone.
This only works for people who want to switch from OTHER carriers. If you want to ditch your own carrier, and sign up again, there is a (logical) penalty for that action of not being able to regain service for 3 months.