my take on it:
brailsford has decided to put all the resource behind wiggins for le tour, and wants cav to win olympic gold. thus, sky will be set up for wiggins in france, whereas payback for cav will be having the team working for him, highroad style, at il giro, to win the sprinters' jersey. he'll probably get to ride le tour for the first week, win a few stages and then 'do a cippolini' as soon as the road gets lumpy, and spend the remaining time before the olympics in preparation.
with a green jersey and two wins on the champs elysees already, and with a long career ahead, it's not hard to see how cav may well consider the forsaking of le tour for one year.
interestingly, the tour of romandie so far suggests that team sky are capable of keeping wiggins high up in the gc as well as delivering cavendish to the line; it's just that the stages haven't suited him. keeping your main contender out of danger at the front is good practice anyway, so why not have the best sprinter in the world there to finish the job off at the end?