There's another thread here,
http://www.greekchat.com/gcforums/sh...d.php?t=120694. It's a trainwreck, but you might be able to get a few nuggets out of it.
My perception is, as far back as the 80's when I was in college (a large Midwestern university so admittedly not exactly the same environment), there were a few to several gay men in a lot of the fraternities, although most were probably on the down low. A guy friend of mine laughingly said there were so many of them, him included, the fraternities should have been having exchanges with each other!
There are also a few threads about lesbians wanting to join traditionally straight sororities. You can probably gain some good advice from those as well.
If it were me, I'd probably feel obliged to out myself before pledging, but as in so many other issues mentioned on this board, you don't need to vomit out your life story in the first 5 minutes of a conversation. I think you can probably gauge the atmosphere of a fraternity by attending their events and have a decent idea of how they would handle the news before spilling the beans.