There are other ways to lock-in exchange rates (forward contracts through Custom House/XETrade, for example, foreign bank accounts (I maintain a EUR account in Germany and a CAD account in Canada), foreign currency, etc.). I would never book a nonrefundable reservation more than about two weeks in advance--due to the chance of rates dropping or a change of plans.

Also bear in mind that the direction of the USD is not a certain thing. It is possible that it will even gain value, especially when the quick drop this week is considered. The Eurozone and the UK have a slew of financial problems of their own.