Here is one from last year reported in the New York Times. It is held up as being very carefully done--reflecting Matt's comment to an earlier post--and conclusive that praying for a stranger did not effect the target of the prayers. (Also reported here.)
This sort of megastudy came out after the article from Sojourner I quoted below. Also, here is a link to an interview with Dr. Larry Dossey, cited by the article as documenting how Western science is catching up in this area.
So, in addition to the value prayer has for the person praying, including the as yet unmentioned therapeutic value of verbalizing what concerns you, prayer may help a person you know while praying for him. I have not yet foreclosed the notion that a person who knows other care for him will be more capable of helping himself heal. Just as I think it is reasonable that a person may "give up" in a struggle against an illness after his spouse dies. (Although, according to Wikipedia, the placebo effect I was hoping to reference my not be real. Ug.)
In moments like these, my dad would say he was letting the spirit move.