From the recently released 2012 Census of Agriculture.
The response rate for the 2012 Census of Agriculture CML is 80.1 percent as compared with a response rate of 85.2 percent for the 2007 Census of Agriculture and 88.0 for the 2002 Census of Agriculture. [More]
These numbers are buried in the Methodology section and surrounded by scary math so I don't think many ag reporters bothered to dig them out.
As near as I can tell (the math scared me a little bit too) the response rate for was pretty even across farm sizes.
Still, we're down to 1 in 5 taking of us a pass. Maybe in 20 years NASS will wonder why, so I can tell them: Too slow, too irrelevant (c'mon - $1000 = farm??)