Um... not the most trustworthy of sources, but....
From The Globe:
Fed up with her hubby's refusal to deal with their shredded relationship, Laura Bush has told the president that she's leaving him for a month---and maybe for good! ... Her desperate decision comes just weeks after a furious George stormed out on her following another bitter argument. "Laura's had it!" says the source. "It looks as if their marriage has reached the bitter end."
[T]he first lady had divorce papers drawn up, and showed them to George, pals say, telling him that if he didn’t shape up she would file. But George has stood by Condi---and now he's paying the price for loyalty to her.
But there is another, confirming source - The National Examiner:
The deteriorating marriage of George and Laura Bush has reached crisis level---First Lady Laura Bush has secretly hired a divorce lawyer, The Examiner can reveal. ...
She remembers what happened to Sharon Bush when she was getting a divorce from George's brother Neil. She didn't get the settlement she deserved until she threatened to expose all the family secrets in a tell-all book. ... "It's over," says the source. "The minute George's term of office expires, so will their marriage."
Does the fact that two tabloids are reporting this make it any more believable?