It's a ploy to quickly get the low leakage effects of 14nm FinFET transistors on a 20nm planar process.
Sounds great for yields (not!). I wonder what it will do to the wafer costs - or does GF intend to propogate their losses by adding cost and cutting prices?
GF mentioned 14+20nm also in their FAQ (incl. the first sentence), which I tweeted on 20th September.
But using 20nm (still not common for ARM etc) with more efficient transistors might be OK. In these days it is not enough to make a smaller chip to save $5 on the BOM of a $600 smartphone, but it should deliver more performance and battery life to better suit the more demanding apps.