Ours expire in September, and I just filled out the renewal forms last night. They'll give you the "fat" ones at no extra charge on renewals if you ask, so I optimistically checked the box for that. Mike's right--they charge a bunch to add pages to the skinny ones. Our old ones only have one or two pages left--we always seem to cut it close but have never needed to get the extra pages.
No doubt Terry would have had to get them in her working days, except she had two passports back then, one for pleasure travel and an "official" passport for her many travels overseas for Uncle Sam. The problem with the official passports back then (don't know if it's still true) is that they were RED. I hated that. She thought I was nuts but I always insisted that she take the "normal" one on business trips too. I just had this vision of a bad guy on a plane looking at everybody's US passports and coming across a RED one. Not the time you want to stand out in a crowd. You'd think they could just put some sort of code in the passport number or something to indicate that it's "official." Maybe they've thought of that by now.
The thing I don't like about renewing is that for the first couple years you look like such a rookie. . .so few stamps. But that's just the whiny complaint of those of us who have been fortunate to see so much of this planet.