The preliminary overnight ratings for World War Three are in: 7.3 million people watched the episode last evening. More importantly, "Doctor Who" shattered its competition: the first installment of ITV's "Celebrity Wrestling" only received 3.8 million viewers, a significant drop (of over 50%) from the previous ITV occupant of the time slot, "Ant and Dec." ViewingFigures doesn't have the regular overnights up yet, for some reason; these figures come from reports on the Outpost Gallifrey Forum by the editors of "Doctor Who Magazine".
Update 24 April 1830 GMT: "Doctor Who" averaged a more precise 7.26 million viewers with an audience share of 38.0%, peaking at 7.90 million viewers during the broadcast. "Celebrity Wrestling" averaged 3.8 million as noted, peaking at 4.3 million, audience share 20.8%. "Doctor Who" in fact had the highest audience share of the whole of Saturday, day and night, and was second only to "Casualty" in total ratings. (Thanks to Ian Levine, Steve Berry, Tom Spilsbury)