I fully expect to see these as either a free pdf update on the GW site (see: Night Spinner) or at least rolled into a print edition at a later date (Battle Missions: Vol 2 or something).
@Wes: I'd love that, but I'm already seeing rumors for too many other new things in a 6th edition codex. Between Warlord tables, fortifications, new units (like flyers for armies that do not yet have them), psychic disciplines, and special Allies tables these books are going to get really big. Not that this is a bad thing. Of course, I also expect that some books will be heavier on some of those and lighter on others.
This is great news if for no other reason than players have the option to see bits of new content before enough of it is done enough to release in book form. Think of it as DLC that will eventually be rolled into the "platinum"/"premium"/"game of the year" edition (generally for much cheaper and available physically than all the bits separately for those who are worried) when there is enough content.