Over at The Raving Theist, there's been a series of posts about friends of his who are dealing with medical issues. One, a child, is facing surgery for cancer. Requests for prayers go out on a regular basis with plenty of folks piling into the comments to pledge their spiritual support.
This week on Dennis Prager's radio program, he was discussing theology with a Jewish physicist and they briefly touched upon the issue of such prayers. Dennis said he wasn't a big believer in it and gave the example of all the people who died under Mao, some 75,000,000 or so, who most likely prayed and yet died anyway. I'd like to suggest that this is short-sighted.
When you pray for someone, it is an act of selflessness. For the moment you are praying, you are subordinating your desires to those of someone else. You are also acknowledging your (very small) place in the world. Whether or not the desired outcome is achieved, you have acted with charity and humility. That in itself is a worthwhile thing.