Many (most?) US airlines now hold back some seat assignments and sell them for a small premium, typically from $10 to $30 per flight. These seats are frequently window and aisle seats in the front of the plane (the first 4-8 rows) and exit row seats. Each airline has its own rules for these seats -- some hold them for their premium frequent flier members, some sell them to anyone, and typically they're sold only at the time online check-in opens, starting 24 hours before the flight.

What can often happen now is what apparently happened to sedona: as seats are booked, the non-premium window and aisle seats all get claimed, so what's left for those booking late are middle seats or the seats with a premium charge. No airline is charging a fee for *all* window and aisle seats... at least yet! ;) If you book your flights well in advance, you can always select window or aisle seats without a fee.

-- Eric