I am in exactly the same situation - going to France and Switzerland this Xmas and considering doing the same. However, I have been internetting about a bit, and also consulted a currency place in London, and the general consensus seems to be that - although not inpossible - the pound is unlikely to weaken any further in the short-term, and if anything, it could push its way up a little over the coming months, especially as the Eurozone is now going to be forced to lower interest rates as they slip into recession.
My personal plan of action is to get half of my money out now, and half nearer the time (getting daily currency updates in the interim so I can keep an eye on things). That way, if rates go up, you haven't completely shot yourself in the foot - and if they go down, you can thank your lucky stars you didn't jump the gun and get loads of money at a worse rate.
And at the end of the day, we have to not worry ourselves too much about it (annoying as it is!) because we are going on holiday, it will be fun and life is too short to worry about currency conversions. Just get what you get, don't even think about the maths and tell yourself you will make the money back again on a future holiday when the pound is back up sky-high and we're on top of the economic world! And eat baked beans for a few weeks in Jan. After all - if you're going to America you'll need a bit of starvation to lose the 7lbs you'll put on while you're there!!