I've done a century before, but largely on flat terrain (bellingham to camano island and back through mt vernon). Is FW 100 miles of rolling 400ft hills or something? Should double crank riders be worried or what?

It is pretty hilly, I did it in a compact double last year with a 12-27 cassette. Expect some pretty long high grade hills.

What are your chainrings/cassette?

I previewed part of the course last Saturday and did the climbs up Ames Lake and 244th. I'm also familiar with the rolling hills on High Bridge & Snoqualmie Valley roads. I haven't ridden anything out by Tolt and Duvall.

Any ideas which hills are considered the worst (steepest or longest)?

Any ideas which hills are considered the worst (steepest or longest)?

Right off the start you get to climb Inglewood Hill. That is a steep mofo and you're barely warmed up.
High Bridge is probably the longest climb, but it's not steep. (It's part of the 7 Hills century route.)
Stillwater and Ames Lake are long climbs, but again, not steep. (Also part of 7 Hills route)

I think those are the worst of them, but seriously the whole route is less than 3900' of climbing.

Yes, Inglewood is the steepest hill on the course. Watch out for people abruptly deciding to dismount without warning on this hill, because, as Clifton said, it's early in the ride and the pack is still bunched up.

The climb up to the plateau on Issaquah Fall City Road isn't that steep, but at that point in the ride (towards the end), it seems to go on forever.

Right on the CBC site is this http://www.cascade.org/EandR/flying/FW_Details.cfm profile.

Inglewood has some 12% pitches.
Stillwater Hill is not quite as steep.
The last one is long ...