The question is engaging the religious. Christopher Hitchens has been diagnosed with cancer. Given his well-known atheism, should a religious-minded well-wisher pray for him?
On the religious side, Rabbi David Wolpe, who has debated Hitchens frequently on religion, puts it very well (as quoted on Goldblog) in my opinion: "I would say it is appropriate and even mandatory to do what one can for another who is sick; and if you believe that praying helps, to pray. It is in any case an expression of one's deep hopes. So yes, I will pray for him, but I will not insult him by asking or implying that he should be grateful for my prayers."
I wish all religious leaders were so open-minded: too often, religious impositions are accompanied by the implication that one should be grateful for the favour, or the threat that one is condemned if one is not grateful.
A scientist on the Dish goes a bit further in asserting that one should not even inform Hitchens (let alone demand his gratitude) that one is praying for him: to do so would be "malicious". In support, he links this randomized trial on the effect of prayer on patients who had undergone coronary artery bypass graft surgery. The study showed that, on patients who did not know whether or not they were being prayed for, prayer had no effect; but patients who knew with certainty that they were being prayed for did significantly worse (exhibited more complications within 30 days of the procedure).
So there you have it. Pray if you like, but don't tell.
(Actually, I'd be surprised if those results were replicable with other ailments: the only explanation that I can think of is that patients who know they are being prayed for believe that their prognosis is particularly poor, and therefore are under more stress -- which is particularly relevant here since they are heart patients. In particular, patients were told, via messages in envelopes, either that they "may or may not be prayed for" or that they "will be prayed for". Perhaps the latter statement was truly frightening to a lot of the patients. I'm unconvinced that the study was ethical: at the minimum, they could have chosen a different ailment, on which stress would not have such a direct and obvious effect.)