I'm on the tarmac in Boston, where we've been for about two hours now. The captain reports that United has centralized the weight and trim calculations for their flights, and that computer is down, so we can't take off, and worse, neither can any other United flight. A quick look around seems to confirm this - any United flight that was supposed to take off between about 9am and now seems to be in the "NOT DEPARTED" state on United's web site. There's no ETA of any kind, though the captain promises not to keep us trapped on the tarmac if this persists.
So if you're flying today, and especially if you're flying United, expect collateral damage.
... just as I'm finishing writing this, the captain says the computer is back up, and it'll be about half an hour before the backlog clears.