Well, the T4G '08 promo video is now online, and towards the end, Mark Dever announces that there may be one additional speaker next year, and C.J. Mahaney calls this a "high probability."
So as Tony Kornheiser would say, "Who ya' got?" Let the rumors and speculation begin here. Guesses? Back-room whisperings? Something you just wanna make up out of thin air? Here's your chance.
[And now, after starting this post, I see that Dever has confirmed that this 8th speaker has been invited, has accepted, and will be announced in a few days.]