It's both good rehabilitation and a decent way to bring more attention to conditions in jails in America - the world leader in locking people up - and across the world. Ethan Zuckerman has a useful round-up of several good prison blogs. And Scott Esposito puts them in a wider context here. Money quote from Zuckerman:

There are reasonable concerns about letting people in jail blog - [Michelle] Malkin references a horrific story about jail writings of a convicted murderer about his crimes being published online to taunt the victim’s family. But I’m not convinced that the story about a very sick man sending letters to another very sick man who posted them should get people to ignore the lessons of Wallen’s success. Helping inmates build skills for the outside world is a part of rehabilitation. And hearing what life is like in prison sheds light on an otherwise dark corner of our world, a corner that’s grown quite huge in modern American life.

More light, please.