My advice: You should research the plans very carefully before entering the store and know exactly what you need/want. Also, check to see if you (or your dad in the OP's case) work for a company that is eligible for a company-discount on the monthly plan rates (and possible waiving the activation fee). I believe this can only be handled in an AT&T store.
If you have experience with another cell company, try to find out the details of your last several months of usage to see if you can get by with a cheaper plan than you might otherwise think you'll need, paying attention to the differences that AT&T's plans have as compared to other cell companies which may help/hurt you depending on your particular situation (e.g., rollover minutes on AT&T vs favorite five on T-Mobile).
My wife and I just switched from Sprint to AT&T. We were on the Sprint SERO plan which cannot be beat by any carrier. I was considering the pre, but would have had to give up that plan, so when comparing the standard Sprint plans, the cost savings were much smaller (and, in our case, amazingly enough, AT&T might actually turn out cheaper). For example, my wife and I had 500 "anytime" minutes each on our old Sprint plan, but only seemed to be using about 200 minutes each. We had unlimited mobile-to-mobile (which AT&T also has), and I'm sure that a lot of our talk time fell into that bucket. Sprint now bundled unlimited SMS into their plans, but we only texted eachother, and not that often, so we used less than 100 messages/month.
So when I bought us two new iPhone 3GS's, I opted for the cheapest family plan from AT&T. It's 550 minutes (shared) w/rollover, and unlimited mobile-to-mobile. I'm torn between the $5/month (per phone) 200 SMS message plan or opting out altogether and trying to get by with just using Beejive (an instant messaging app that uses data and Apple's new push feature, and doesn't use SMS at all). My wife also works for a company that gets a 12% discount on the monthly rates.
So, all told, if I'm doing my math correctly, I think we'll be paying about $106/month. That's about $40/month more than we pay with our old Sprint SERO plan, *BUT* if we bought two Palm pre's on Sprint, we would have had to switch plans, and the cheapest family plan would have been $130/month (we should be eligible for company discounts there, too, but the end result would have probably ended up about the same or higher. We would have gotten more minutes and unlimited SMS on Sprint, but we don't really need it, so it's a wash.