Abraham's prayer is unspoken, but he prays it through his service. He doesn't just welcome the visitors that come to his camp; he gives them the best that he can offer. In return, he is rewarded with the promise of a son, a treasure beyond any means that he had to provide.
People looking at today's Gospel can easily dismiss Martha as being too involved in worldly affairs as she complains to Jesus about the lack of help that her sister is giving her, but (as our newly ordained priest pointed out) her conversation with Jesus also shows us about the nature of prayer. First, she is open and honest with Jesus. She speaks with Him frankly about her concern and does not hold back. Secondly, she clearly believes that He has the power to help her. And last, it shows that Jesus does not always answer the way that we might want or expect.
I've read a lot of posts these last few years about the futility of prayer. I beg to disagree with all of them. Prayer connects us to our world through God. If done correctly (openly, with belief, and the resolve to act), prayer grants us treasures that are beyond our own means to provide. We may not get the answer that we want, but God will provide the answer that we need.