In regards to a "General Rule".
Every time that I've read of a housed national fraternity or sorority accepting someone working on a graduate degree it tends to be a "one off" situation, where just because this chapter did it doesn't mean that other fraternities/sororities on that campus *or* other chapters of the same fraternity/sororities would be likely to.
See
http://www.greekchat.com/gcforums/sh...ad.php?t=67042 for a discussion of this in regards to pleding a sorority. I have *no* idea if the fraternities would be similar.
As noted in the other thread, a high number of Alpha Phi Omega (co-ed service fraternity/non-housed) chapters allow graduate students.