Red State has updated and said the retirement has occurred......this is not confirmed by Drudge (who seems to be backing off the story) or any other place:
Red state update: "Update [2005-7-8 17:20:42 by Erick]: William Rehnquist, Chief Justice of the United States, has retired. A White House statement is forthcoming. Should caveat this: A very good source says this."
He also said 2 other sources told him that, though no WH sources.
I think that source is right that he will be retiring and wouldn't surprise me that the President gets his letter upon return to the White House. That said, I am going out to finish mowing my lawn which means the announcement will occur momentarily since I delayed mowing to hear! :)