I really want to say that JFK was a Phi Kappa Theta, I think because of the Catholic thing. One of my profs is an honorary initiate of the same fraternity - he was their chapter advisor for many years and they decided he was worthy of initiation, which I agree with them on - if he was a woman, I'd want him to be a Gamma Phi Beta!
I know Bill Clinton was an Alpha Phi Omega, so he did get a bid somewhere.
As for the practice in general, I tend to agree with DeltAlum - it's like "Oh look, we don't have famous alumnae, let's go get some!" I do think it's acceptable in some situations. For example, Dr. Laurel Clark, one of our prominent alumnae, died in the shuttle disaster. If several of her fellow astronauts were interested in Gamma Phi Beta, I would see no problem with that - they have a connection. I think it would be wrong to just call up, say, Mandy Moore, and initiate her for the heck of it. If her best friend was a Gamma Phi, I think it'd be okay?
Does that make sense?