It seems Xilinx has already released some tools for Kintex-8 and Virtex-8. Maybe even chips are in the wild. I wonder if they are 20nm parts. It would be so nice to get an eval board if they do exist. One reference to Virtex-8 is here: http://www.xilinx.com/innovation/research-labs/keynotes/RAW2012_keynote.pdf

Here is a report from Digitimes which says 8 series parts will be manufactured on TSMC 20nm process: http://www.digitimes.com/news/a20121115PB200.html