The split/alternate thing's been all over DWF and elsewhere.

I'm pretty sure the Racnoss isn't back as a proper baddie, we're just going to see what would have happened had Donna not travelled with the Doctor and become who she is today and such - more like the re-filmed "flashbacks" Elton had in L&M of the Autons etc than anything bigger. Sliding Doors. Blah blah. There's something on your back...