Here is some back of the napkin numbers...

Ken estimates 1-2PH.

Current design indicates that the base unit holds 6 cards and expansion units hold 15 cards.

Current labels show a maximum build out of 1 base unit + 6 expansions for a total of 24.576TH.

Extrapolating that downward we can see that each card has 16 chips and at our new 55nm specs that equates to around 32GH per card. This means that your initial 4U produces 192GH and each expansion roughly 480GH. For a net total of 3TH per 28U of rackspace.

Now it's certainly possible that the boards are redesigned to hold more chips; simply can't say. Now there are some other legistics problems in a data center... how long are the interconnect cables? Can an expansion shelf be in a different rack than the head unit? Tossing all of that asside for a moment... at 3TH per 28U you are looking at 9334U for 1PH or 222 racks (@ 42U per rack) *not accounting for spread, network gear, etc.

Or if you want to assume worse case and go with 1 Set (1 head unit + 6 expansion) per rack you are at around 334 racks.

But all of that aside; the real challenge isn't getting power... it's going to be in the cooling at least in my opinion.

*** Assuming I did my maths right ***