It appears his trip -- as a private citizen -- is designed to secure the release of an American citizen.
If Mr. Carter does make this trip, he will, as the aforementioned link notes, become the second U.S. president to make a humanitarian trip to Pyongyang in as many years. President Clinton made a visit there last year, when he did secure the release of two Current TV journalists.
The inevitable question that comes up in such situations is whether America's former presidents should be used in this way. Despite the "they're going as a private citizen" rationale, there is little question these men are making such a visit as a representative of the U.S. government.
Moreover, they're called upon when it appears no other avenue for "rescue" has proven successful. They will meet with -- and certainly be required to take photos with -- North Korea's dictator Kim Jong Il. Such a photo provides rich propaganda value for a regime that treats its people with disdain.
Mr. Carter will, in my opinion, be remembered more for his humanitarian work once he left the White House then for his single term in it. I'm not sure that traveling to Pyongyang and returning with an American citizen will bolster his credentials, but I am sure that there will be vocal critics insisting that the government is sending Mr. Carter on a mission that is akin to talking to terrorists.