I was working on a question about agricultural land sales. The LEXIS Deed Transfer file covers all NY counties and includes a field that denotes land use. Using agricultural as the keyword, and inserted a date range, you'll get over 3000 results for NY for one year so you'll need to add the county name to the search. The records also include the number of acres and sale price.
www.farmlandinfo.org has info by state and includes some limited statistics and some farmland protection plans (some of those include some statistics). Within that site, you can change the state in dropdown. Lists ag land converted to developed use 92-97; also see Recommended Data Sources at bottom, then National Resources Inventory (2003) has Land Cover/Use by state.
This site is PA only. Data is by county, gives number of farms/land in various years.
Of course, there's the Census of Agriculture. This table shows Farms, Land in Farms, Value of Land and Buildings, and Land Use for 2002 and 1997. The 2007 stats won't be out until 2009 (they will be sending surveys out to farmers this month). This table is for NY only. There should be similar ones for each state. To access go to http://www.nass.usda.gov/ and see "Statistics by State" at bottom of left column.
Wyoming county projections/land use trends. Is there more about other counties on this site?
"Rural Landowner Survey 2005" for NY is available on the Ag and Mkts site.
In the NYS Statistical Yearbook, 2006, there is a table of Farm Acreage by Land Use (table N-3, page 559). The same table is available here; click on Farmland Use.
Thanks to the New York State Library for much of this information.