The next Magic novel appeared on Amazon for pre-order and it seems to confirm that we will see Karn in the next block. "Scars of Mirrodin: The Quest for Karn: Scars of Mirrodin Block" (link)

Will this be the first colorless / artifact Planeswalker as well as being the first previous Planeswalker creature to be reprinted as a Planeswalker? That is yet to be seen. If true, Karn would be appearing alongside Tezzeret and Elspeth for the block.

Update: Removed the obviously stupid statement about ex-creature being reprinted as a Planeswalker. The original author has been taken out back and shot.