I stand corrected:
Full Frame DSLR Cameras Part I ? Nikon vs Sony | Technology Blog
The D700/D3 were probably not made by a Sony foundry but by Renesas with Nikon design to the core.
The D600 and D800 are Sony sensor, likely using Nikon design input.
The Sony a850/900 could not compete with Nikon's designs, so Sony, eager to keep Nikon biz, have obviously come to some arrangement now being the main foundry for Nikon FF, the D4 excepted (but being a 1/2 generation older than the D800/600). Sony's not competing on the "pure" DSLR format going with SLT/EVF and the RX compact.
Pentax FF availability relies on a price-competitive sensor in MP's and ISO. They can approach Sony whose products are the clear front runner in IQ but for this to happen Sony as #3 in FF (I'll leave Leica bling out of this) they'd have to make room for a #4 in such a way as to grow the overall market and not cannibalize sale either from Sony itself or from their largest customer by far, Nikon. Even with the 645D under their belts, Pentax/Ricoh does not appear to have the resources to design a spec and price competitive, mass produced FF sensor.
I am still not seeing FF sensors with commodity availability and price as APS-C. Once sensors reach a commodity status then shopping around is common, such as Canon using Sony sensors in compacts, and Fuji doing their own thing. As long as FF sensors straddle the line with being custom designs and multiple points of redundancy (D4/800/600) to capture pro and prosumer market, the volume factor in sales becomes the driving force. For Pentax to succeed at FF, they literally would have to replace the K-5 line entirely with FF at a near-equivalent price and lens availability, all at once, while still maintaining a very good APS-C offering, bodies and lenses, just for cash flow.