This article which states that the prime minister of North Korea has been replaced by the former army and navy minister, Kim Yong-il. The problem is, I do not feel I know enough about the internal structure of the DPRK to really understand what that means. The Wikipedia entry on Pak Pong Ju gives very little information about him. He appears to have consistently gone outside Korea, to China and other places.
This article from Asia Times indicates that Pak was considered by outside watchers to be something of a reformer, though in a limited way allowed to him in Pyongyang. However, this article was from about two years ago. I have not managed an exhaustive search, but very little has been written about him which is unsurprising given the secretiveness of his government.
As far as I could tell, nothing is written about his replacement, Kim Yong Il.
Anyone who knows more about how the North Korean government works and wants to speculate on why Pak was removed? Has becoming prime minister, an automatically risky position, simply finally caught up with him? Or is it related to the Six Party Talks? Or simply to continuing economic difficulties? Could he possibly be a threat in the power structure? It will be interesting to see if North Korea gives any indication as to why, and what analysts say about it.