Andy needs time, Andy knows he needs time. He's taking a terrible risk if he waits until stage 17 to the Tourmalet to take it, especially as it comes after the rest day. Stage 16 has too much flat after the last summit and Astana look too strong for an early attack to work, so tomorrow looks like an opportunity he cannot afford to pass up.
He might be able to do it, too. Contador hasn't looked weak, per se, but I think it's fair to say he hasn't looked as strong as he did last year. I've grown used to seeing Alberto drop absolutely everyone in the high mountains, pretty much at will, but when he attacked on the final climb today not only was Andy able to stay with him every single time, but Denis Menchov even managed to hang on once or twice. If Andy sees even a hint of a chink in Contador's armour tomorrow he needs to attack.
[edit]This word filter situation is OUT OF CONTROL. Chink is a legitimate word, I haven't heard anyone using it as a racial slur since I left primary school.