i know how you feel, been there done that.
it doesn't hurt to call them to ask why you failed. of course whether they tell you is another story, but i am sure they won't mind that you call.
sometimes you can fail interviews because of some presentation problems. your answers may have, due to one reason or another, failed to make their mark on the interviewer. do remember that they have to interview many people per day and when it's time to make a decision on who to pass and who to fail, you want to be remembered by the impression you gave them. if they don't remember you, you're out, no matter how qualified you may seem. this was my diagnosis of my own problem, hope it can be of a small use to you.
cheer up pal... it isn't over yet, not by a long straw!
