Based on my high level analysis of the data, of course with quite a few asssumptions, it seems that PD of Dec 05 will be current around August of this year!!

I-485 inventory data as of Jan 2011 and Visa bulletin as of April 2011:

a) Total I-485 in Inventory: (July 05 and prior): 11,456
b) Total number of visas availble in FY 2011 for EB3 ROW: 30,000 (it's 28.6% 140K, but not all goes to EB3 ROW; about 5K goes to EB3 India and China and another 5K goes to EB3 Other )
c) Approved I-485 (linear assumption, so 30K/12= 2500 every month)
d) Remaining quota for FY 2011 = 15K, about half of the fiscal year has already gone by! 
e) I-485 Inventory till the end of 2005 is 9K (add 1K for people who missed the July 07 filing period), so total 10K

So, with 2500 approvals a month, it will roughly take another 4 months for December 2005 PD to be Current! Hence August!

For what it is worth, my long term forecast: 
End of FY 2011: PD of March 2006
End of FY 2012: PD of October 2007
End of FY 2013: PD of February 2009
End of FY 2014: PD of June 2010
End of FY 2015: PD of July 2011