I can take a stab at C.H's challenge:
If a god said, (hypothetically speaking) he would send everyone to heave if the atheist believed in him?
If the athiests accepts yes, then he would no longer be and atheist and would only be doing it as a believer, if he did not, then its only something (which is good) can be done by a believer.
Where's my check Mr Hitchens?
Can you define the challenge?
Just watch the video he mentions it. I think he mentions it at the end too, first question at the end of the first question at the end. He also mentions its somewhere in the mid-section.
Its something along the lines of, is there even one thing which would be considered ethical or moral only a theist could do, and an infidel could not do, he said name even one universally accepted morally good thing, only a theist could do, which would be defined by his deity.
Best to watch the video though.