There are two primary reasons for the change from "H" to "S" over Maine, as follows:

First, the estimated location of the center of Irene during the final period of the 5-Day Cone of Uncertainty from Advisory 18 is now further inland than shown in Advisory 17A , and as such, Irene will have had more time inland weakening, essentially as Random C mentioned above. You can run a historic loop of the graphic Advisories here: http://www.nhc.noaa.gov/archive/2011/graphics/al09/loop_5W.shtml

Second, and related to the first reason, Irene is officially forecast to have turned post-tropical by 2PM Monday. However, it is worth noting that this forecast is largely dependent on Irene being well inland at that time. Should Irene still be partly, largely, or fully over water, it is very possible that she would be more tropical than not, and as such, may still be a hurricane as of Monday afternoon.

As to what New England should expect, it is too early to tell for sure. Tropical Cyclone forecasts beyond 3 days are notoriously off by up to several hundred miles. As during all seasons, it is best to prepare for the worst and expect the best. Irene has the potential to be an epic east coast storm. Potential.