Relax. It's mid-August. Few people pay attention to the campaign - particularly with the Olympics starting off. It's the best time to refill one's batteries.
Obama has sure had a gruelling schedule for months on end, working on weekends etc. He more than deserves this holiday and the one he took earlier this year.
Funnily, you wouldn't even really notice if McCain were to go on holiday. He just doesn't get nearly the same sort of attention (which both helps and hurts him). Anyway, McCain seems to take off more weekends rather than taking holidays en bloc. But then he doesn't have small kids.
It's important for candidates to be in good shape in high summer so they can shine during the campaign and in the following three months. I dare say his campaign would beg him to take the holiday weren't he doing so.