It's in retaliation for a United sale into Dallas and St Louis if anyone is going (to or from) there. (
tomwcarr, it might be a good day to check fares for a lot of future travel on United if you're happy enough to use them...)
For those who track airfares, there's a new site useful for the purpose:
http://www.farecompare.com. You can get a map with the cheapest flights from your home city to most major US and international airports. It's similar to Travelocity's 'Dream Maps' if you've used that, but much more functional. It will also give you graphs of historical pricing by route so you know whether you're getting a good deal or not, and therefore whether it would be better to buy or wait (if you're feeling lucky).
For example,
here's the map with Boston as the center and US destinations. And
this and
this are graphs showing that the United $140 fare from Boston to St Louis posted this morning is - really annoyingly - an outstanding fare - but unfortunately it's not valid for less than 7 day advance purchase, so no last minute team trialling for me!
Note: the site appears to use cookies in such a way that deep links may not work. If the above links just redirect you to the homepage, sorry.