I want to transfer an old maildir from my previous linux box to my new install. Is it as simple as cp'ing the dir across or is there some archiving/syncing that needs to be done first?

If it's mbox, then you just copy the file. If it's Maildir, you just copy the directory. Other formats, I don't know. You probably want to ensure that nothing's trying to deliver to it at the time, though.