What is the highest recorded difficulty in the main net? What is the highest recorded difficulty in the main net and which blocks was it used for?
As at 9th May 2012, the highest recorded difficult has been:

1,888,786.705353 first reached August 01, 2011.

Blocks 139,104 through 141,119:


http://blockexplorer.com/b/139104
Bitcoin Difficulty History