I have this problem with Harry all the time. Generally he's good but has these moments where nothing really seems to sooth him. Walking round with him and 'shhing' in to his ear occasionally helps. Apparantly it's suppost to imitate the noise of the mothers heartbeat when they're in the womb.
When he throws a wobbly in his moses basket, he stops crying if I take him in bed with me, but I'm a bit worried about getting him in to that habit!
I read in a book somewhere that it could be just attention seeking. If the cry's are a bit shouty, ratehr than screams, they just want to be played with!