This thread was just in time for me. I'm flying 644 -> 450 Sunday with an hour connection time, so I was a bit concerned. The flight from IAD to KEF seems to be delayed often, but when I checked the flight from KEF to LHR it's almost always late -- often by hours. So I suspected they hold the planes for connections.

I'm not really that worried about arriving in London late.... as long as it's not a day late that is!