If you need to extract all mails from a single mbox file to seperate .eml files, take a look at this tool. I had recently the need to do so, and this tool was just perfect for it. It lets you choose which files to extract from mbox to .eml. In order to import a normal GNU/Linux (any *nix?) mbox into the tool, you need to set as file-ending .mbx. The Only restriction is that it can only handle western character sets, but not sure if that’s a big problem, aren’t most Emails still in latin?
To run the tool you need to have installed Java Runtime 1.2 or higher