I think it depends in part on the time of year, whether you're flying midweek or peak travel days and how far in advance you book.

We're traveling SEA-SFO-PEK//HKG-SFO-SEA on United in late March returning in early April (midweek travel, 10 day trip). Booked Aug '09 on an upgradeable fare for $950.

In early June, United is showing some midweek fares as low as $1,160 and Expedia is showing some fares on Delta around the $1,300 range - BUT all this can vary greatly if you leave on a Weds versus leaving on a Sat...