The evidence, which in an ideal world would end his Presidency, after the jump... 
The short version? The White House has long denied Bush knew anything about the military's/CIA's policy of deliberately torturing and abusing inmates. Although that signature does not appear to say "George Bush," sources say it is his handwriting and that this is no fake.
UPDATE: Did anyone see Andrew Sullivan on TV last weekend? Well, he basically said that Bush administration officials should remain in the country, since war crimes charges are possible. And he's a gay Republican-- imagine what the Democrats will say on the Sunday shows.