I was unaware until now that the sequence of impact was north tower, south tower, but the sequence of collapse was south tower, north tower:

8:45 a.m.: (all times are EDT): American Airlines Flight 11 out of Boston, Massachusetts, crashes into the north tower of the World Trade Center.

9:03 a.m.: A second hijacked airliner, United Airlines Flight 175 from Boston, crashes into the south tower of the World Trade Center.

10:05 a.m.: The south tower of the World Trade Center collapses, plummeting into the streets below.

10:28 a.m.: The World Trade Center's north tower collapses from the top down as if it were being peeled apart.

source: CNN (warning: flash) (link w/invective via mantid on MetaFilter)