Nvidia and AMD are more likely to use 20nm before Apple is. Apple needs a high, consistent supply of chips in the millions, where as Nvidia and AMD sell $500 GPUs that can withstand supply shortages for a while. Also, looking at release schedules, a 20nm Apple SoC probably wouldn't ship until mid-2014.It also depends when TSMC actually get 20nm.
Plus we know Apple will outbid as well. Might be early 2015 for GPUs.