We actually do have AI, usually a few women per year. One of the most notable recent AI's was the Washington/Oregon President/CEO of the National Kidney Foundation. There was a very nice write up about it in the "PhiSignificant" section of the Sphinx in January.
AI is usually done in special circumstances, for women who are leaders in their community, OR in cases where they are needed to help a specific chapter... like I believe one of the advisors for our Iota Zeta chapter was an AI.
Just an FYI