We have no problem with this. There are literally thousands of women who are members of NPHC sororities that are also Iota Sweethearts, some of whom joined their sorority first.
As stated on our website at
www.iotaphitheta.org/sweethearts.htm, Iota Sweethearts are not to function as or to be construed as a substitute for a sorority.
Because they serve different purposes, we do not consider the two to be mutually-exclusive.
And for those who may be wondering, you can consider this to be the official word from Nationals on this subject. 
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IotaNet
Iota Phi Theta Fraternity, Inc.
Alpha Eta Chapter, Spring, 1980
Kappa Kappa Psi Honorary Band Fraternity
Zeta Nu Chapter, Spring, 1979
[This message has been edited by IotaNet (edited August 07, 2001).]