Connection in Seattle
 

We will travel from Copenhagen to San Francisco with SAS. The itinerary is CPH-SEA-SFO. We arrive at SEA at 4:50PM (16.50). Then we connect to SAS (SK 8803) departing for SFO at 7:00PM (19.00).
Question: Where will we have to go through passport control -- in Seattle or in SF? If Seattle: After passport control, will we have to retrieve our suitcases, go through customs and recheck them –- how long will this take -- or will our suitcases be checked through to SFO?

You'll go through in SEA, and will have to claim you bags and recheck them.

It's a (pretty much) painless process in SEA and you should have plenty of time.

You should also be aware that SK 8803 is a codeshare flight operated by United Airlines.

As alanw noted, you will clear C&I at Seattle.

You will be arriving at the South Terminal (S Gates) and your flight to SFO will be departing from the North Terminal (N Gates) at the other end of the airport.

If you are a UA 1K or 1P, you can use the elite security line for the North Checkpoint, otherwise you might be better off re-clearing security at the South Checkpoint and take the shuttle tram from Main Terminal South Station to Main Terminal North Station and connect to the tram to the North Terminal.

The North Checkpoint can get very busy, but not a lot of airlines use the South one (and most that do are international with late night flights), so I find it can be faster.

SEA Map - http://www.portseattle.org/about/maps/transit.shtml

Unfortunately your SAS flight will arrive around a half hour after British Airways' nonstop 747 from London, which tends to run very full. If the BA flight is delayed at all, or yours is early (both possible) then the crowd at immigration and customs - which is not a very big station - can be rather large. Nothing you can do about it, but I would not delay getting off the plane or into the lineup for Immigration.

Past that, you should be fine, with adequate time.

Welcome to FT by the way.