Orchestras that would welcome non-music majors and would also challenge serious players.
Here are suggestions from the NACAC listserv (intially compiled by Dodge Johnson in November of 2005):
Amherst
Austin
Azuza Pacific
Baldwin-Wallace
Bard
Boston University
Brown
Centre
Chicago, University of
Claremont Mc-Kenna
Columbia
Cornell
Davidson
DePauw
Emory
Furman
George Mason
Gettysburg
Hampshire
Harvey Mudd
Johns Hopkins
Kenyon
Lawrence
Lehigh
Macalester
Massachusetts, University of
Middlebury
Moravian
Mt. Holyoke
Oberlin
Pacific, University of the
Pitzer
Pomona
Potsdam, SUNY College at
Puget Sound
Rhode Island School of Design (through Brown)
Rhodes
Rochester
Scripps
Skidmore
Smith
St. Olaf
Stanford
Swarthmore
Texas State, San Marcos
Tufts
Vanderbilt
Vassar
Vermont, University of
Wake Forest
Washington University
Wesleyan
Wheaton (MA)
Whitman
Willamette
Williams (audition for Berkshire Symphony)
Wooster, College of
Yale