Until today, NASA was predicting its Upper Atmosphere Research Satellite (UARS) would strike on Saturday, September 24... plus or minus one day. Turns out, it's minus. NASA's most recent projection is that the bus-size satellite will begin re-entry and hit the surface Friday, September 23.
The satellite will leave orbit and begin re-entry about a day earlier than NASA had originally projected, albeit within the earliest part of NASA's recently published three-day window for the event. The change in projection came about because of an unusual amount of recent sun activity, flares and sun-storms, heating the upper atmosphere. Heating expands the atmosphere, which increases drag on satellites in low-earth orbit, be they dead or alive. Live satellites have fuel to use to boost them back into proper orbit. Dead satellites, of which UARS is one, have no fuel remaining; meaning their degrading orbits degrade ever more rapidly as the atmosphere expands.
UARS masses 6.5 tons, of which NASA estimates 1,100 to 2,000 pounds will survive to reach Earth's surface. NASA projects that at least 26 pieces of the hulk, varying in size and configuration, will strike the surface, land or water, in a strew-field 400 to 500 miles long by as many as 50 miles wide. Irregularly shaped pieces of various sizes tumbling through the atmosphere and sloughing melted metal will change lateral direction as they go, but only by a limited amount.
The general land-strike location (the 500 mile long strew-field) will not be projectable until about two hours before re-entry. Given the number of pieces and the size of the strew-field, NASA estimates the chances of a piece of UARS striking a person at 1 in 3,200.