Well, tried out an Alpha a77 with my Sigma lenses today (Bigma and 24-70 F/2.8 EX DG Macro - both from 2006) and, wouldn't you know it, both worked absolutely fine.
I now have only one excuse left for not buying this camera, money. Unfortunately, I CAN afford it at the moment, hmmm....
Brief impressions of the camera:
Lovely to handle
EVF is AMAZING!! I was very worried about this but needlessly as it turns out.
LOTS of functions (I think that there may be a bit of a learning curve until I'm as familiar as I am with my 7D)
The 12fps is just silly
The 16-50 lens is lovely too, compact, super silent focus, fast, etc., etc.
It feels like a "real" camera.
I'll see how long it is before I crack.