Does anyone know how it works if you want to use miles for a multi-city ticket? Specifically, I'd be looking at doing PHL-SEA-SFO-PHL. The PHL-SEA and SFO-PHL portions would be on US metal; SEA-SFO would have to be on UA. I wasn't able to find anything on the US website on this. Thanks!

Looks like it's by giving them a call. You probably already tried this, but if you click on the 'One-way and multi-city award travel' link in the 'redeem my miles area' on the site, a 'One-way and multi-city award travel reservations can only be made by calling our 24-hour Reservations Center at 800-428-4322' message comes up.

I know it's out of the way, but maybe ask about connecting through Vegas between Seattle and San Fran. If you want to skip using UA and if you have some extra time, maybe they'll work with you to arrange a 14-hour-or-so 'layover' in Vegas between your connections...

Let us know how it all pans out!