My extended weekend trip to Paris in July will be paid for in BA miles (Concorde) and Hyatt Gold Passport points, so other than a little shopping and my meals, I don't think that my trip will be considerably more expensive than where I will be coming from, having been in New York working the week prior.
While the devaluation would be of consideration for a longer trip, I certainly noticed a change in value between my 2000 to 2002 trips to Paris.
I wouldn't postpone a trip to Europe due to the weak dollar, as others have noted, there probably will be less crowds to contend with and perhaps, prices will come down in hopes of luring more tourists to visit the many wonderful European cities.