HT to Jimgotkp on FlyerTalk for sharing this deal from Seattle to Boston on American Airlines. You can route through DFW with a single stop in each direction, as he suggested, and get the price down to $260 and 4 CPM. But I was able to include additional stops in LAX and ORD for only slightly more. The final cost is $268 and earns 7,065 miles for an average cost per mile of 3.8 cents. There is lots of availability, at least in February.
Note that an overnight stay in Boston seems to be required — and Boston area hotels aren’t especially cheap. A more thorough search might be able to eliminate this need if you can find a satisfactory red-eye flight. Reports are that it is also available in reverse, and for as low as $238 on Alaska Airlines.
UPDATE: bmvaughn shares that it’s possible to get the non-stop on Alaska Airlines for only $218, but I see fewer dates available. That brings it pretty close to mileage run territory (4.38 CPM) for Alaska, which is not known for such cheap fares on its own routes.