That is a tough one, the rates have never been this bad, I would be tempted to hold out and just see if there is a pattern of them going up at all, they did go up a tiny fraction when the UK interest rates came down but they've gone down again a bit since then.
It's a hard one to call but if it does go up and you get your dollars now you'll be kicking yourself that you didn't hold out a bit longer, even if there's no real change then at least you haven't really lost or gained much by holding on.
Not that helpful I know.....