Originally Posted by Sneezy
Not totally true. Once you're within 24 hours of flight time and can check in online, you can go through the online checkin process, change your seats, then click on the "Log out" link.
Your newly selected seats will be saved, even if you don't actually check in.
Excellent point, and an omission on my part. But it's not nearly as user-friendly as, say, AA.com, which let's you see the seat map anytime and switch your seats willy nilly.