I'd suggest that this is not as a far call as you would think.
If we look at the Nikon side, it is quite probable that the 3000$ D800 and not the D4 that is the image quality champ. If you check DxO mark the cameras are close, and it is hard to ignore the added resolution. On the other hand the D4 outperforms the D7000 and the D3200.
I have used the A900 and the Alpha 77 SLT in parallel. For action I would use the Alpha 77, any time. It has better AF, live view and also less vibration as it has no moving mirror.
Once you take DoF into consideration, there are some benefits to a smaller sensor. An APS-C sensor gives you 1.5 - 2 apertures for similar DoF. This really compensates for some of the advantages of the larger sensor. Another factor is that an APS-C camera with a APS-C lens much smaller than a full frame camera with a full size lens. Shooting on the street I would much prefer my Sony Alpha 77 with a small 16-80/3.5-4.5 than shooting with the Alpha 900 and it's big and heavy 24-70/2.8. Image quality is pretty close between the two.
When shooting landscape and similar on tripod I would normally use the Sony Alpha 900.
If we return to Nikon, the only full frame Nikon I ever would consider is the D800, in my view the "flagship" D4 is clearly an inferior camera as it has half the resolution. Would I need high frame rate and a camera that can be bumped around, it would be a different thing. But for a landscape shooter using tripod I have little doubt that the Nikon is the king of the hill.
I did consider switching from Sony to Nikon when the D800 arrived, but I did not. Could be smarter to have done that.
Interesting that there should even be a debate about the relative merits of a $1050 2011 APS camera and a $5500 flagship FF camera.
And just in the interests of disclosure I bought my first SLR a Minolta SR-T 101 in 1971 and have had other Minoltas, a number of Nikons ,and also Olympus, Canon and a couple of Contax. I have owned a number of Nikon DX cameras in the past but the Sony A77 is my only current small sensor DSLR and is likely to be my last as I have bought a Nex . During the 1990's I did decide to go Nikon and reduced my Minolta kit accordingly until Minolta introduced the Dynax 7. I do have a small secondary Nikon kit based on a D700 but my main cameras are the Sony A850 (tripod), the A77 (hand help mainly local sports and events)and the Nex 7.
I think I will wait out the 2012 releases and see what 2014 brings. Not to say that I'm not in the market for any new lenses that really, really appeal to me.