Members of the Association of American Universities:
Big Ten = 11 (all of them)
Big XII = 7 (UT, A&M, KU, Mizzou, CU, Neb, Iowa St)
Pac 10 = 7 (Wash, Oregon, Arizona, UCLA, UCB, Stanford, USC)
Ivy League = 7 (all but Dartmouth)
ACC = 4 (UVA, Maryland, UNC, Duke)
Big East = 3 (Syracuse, Rutgers, Pitt)
SEC = 2 (Vandy, Florida - surprisingly, no Arkansas)
I started looking into it just to compare UT and Arkansas - but I found it very interesting to look at all the numbers. I'm actually a bit surprised that 5/6 Big XII North schools are members. Four of the Pac 10 schools were added since 1969.
It probably doesn't mean much to look at conferences this way, but these are considered to be the 60 leading research universities in the US (2 in Canada).