From the WS 2003 application:

Road Qualifying Times:
Age           50 miles  100 km  100 mi
39 and under  9:00      12:00   24:00
40-49         9:30      12:30   24:00
50 and up    10:00      13:00   24:00
One qualifying run at any of the above distances and times will qualify you for WS.

Notes:
(1) If you are using a trail run as your qualifier, one hour can be added to these times.
(2) If you completed any official* 100-mile trail races in the previous year within the 
    maximum allowable time of that particular race, that race can be used as a qualifier.
(3) Three 50-mile runs completed within the cut-off times for those races, but not more 
    than 12 hours, and during the qualifying period, can also be used.
(4) See the entry form for additional details.

*Results must be listed in Ultrarunning Magazine.

The qualifying period is October 1, 2001 to November 11, 2002.  Entry with
proof of qualification must be received (not mailed) by November 15, 2002.
The lottery is held December 7, 2002 at 9am.  The race is June 28, 2003.

The eight (8) hours of trail work or volunteering at an ultra must occur
between June 1, 2002 and June 1, 2003 and be returned by June 1, 2003.

(updated: 7/25/02)
http://www.Run100s.com/WS/wsqualify.txt