Schwarm:

These are UA flight numbers on UA metal -- its just my elite status that is from US. I was able to get seats on the shorter flights between IAD and YUL (on the Embraer 170), its just the transcons I could not. It was the note on the itinerary for just the SEA-IAD flights (even one 8 days out) "All seats this flight assigned at airport only" that surprised me, especially the word "all" -- as if everyone got their seat assigned day of flight for this route (like a number of European carriers do for intr-Europe flights).

Thanks for your response.