While Mark Rosewater wasn't offering any spoilers from Battle for Zendikar, he did announce on his Tumblr that starting with BFZ there will be no more seeded packs are pre-releases.

The Battle for Zendikar preview weeks don’t start until September, but I’m not a heartless man. I’ll share a little Battle for Zendikar news with you all today.

Starting with the Battle for Zendikar prerelease, we will no longer be using seeded packs. You will get six Battle for Zendikar boosters along with a stamped preview card that can be any rare or mythic rare from the set. This change was made based on a lot feedback from all of you.

I now return you to your waiting for Battle for Zendikar previews. : )

This is an interesting decision to me. For one, prereleases weren't very good testing for actual sealed with seeded packs offering "training wheels", but this only matters if the prerelease is something that we want to be competitive. Given the crowds that I see at these events, I don't think this is the case. At any rate, this seems to be what players say they want, and I like it.

It will be interesting to see how having any rare or mythic from the set as your card pans out. The prerelease cards of late have been valued between a non-foil and a foil, and this will presumably make the prerelease cards worth closer to the value of a regular pack foil.