Again, Canonfire is your best bet for cheap info. One particular piece I can point you to is an article in one of the Best of Greyhawk PDF's (downloadable from Canonfire), issue #9.
Also, I think the next issue of the Living Greyhawk Gazetteer in Polyhedron is going to have an article by Gary Holian about paladins of various gods in Greyhawk and I think Rao might get a mention.
Now if you're willing to hunt references and pay some cash then I'd suggest getting one of the OOP Grreyhawk books, namely The Marklands which deals with Veluna, the nation that holds Rao as the state religion and where the Crook of Rao, potentially the most powerful artifact in existance on ANY world (which is why some people think it's so lame, that and there was never an adventure made to retrieve it, it was just said, "And lo, it was welt, and thusly the fiends were smoten!").
Umm... what else... OH, YEAH! I almost forgot, today is Greychat! Come to chat.psionics.net (the same network the EN World's chat room is on) and join the channel #Greytalk. I think the chat starts around 8 or 9 EST though I'm not American so I'm not sure about the US start time.