This article describes the U.S. response thus far and sheds some light on the relationship between the Afghan judiciary--described as "a bastion of conservative orthodoxy, largely unreformed despite the ouster of the Taliban more than four years ago"--and the Kabul regime.
Afghan Constitution, I should note, explicitly gives the President the power to reduce penalties and requires his approval for carrying out a death sentence.
Update: This guy has translated comments on the case from the Al Arabiya website.
Update #2: According to this story, the Afghan government is looking for a way out, perhaps by claiming that Mr. Rahman is mentally ill and hence not legally accountable for his apostasy. This smacks of the old Soviet move--unhappy with our communist utopia? you must be crazy!--and I’d prefer a bolder affirmation of religious freedom in a Muslim context. But it’s a start.