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:
As the comments mention this value is likely to change over time. Since Stephen Gornick's answer in May 2012 the highest recorded difficulty is:
At the time of writing (24th November 2012) this is the current difficulty and the difficulty is expected to increase again in ~257 blocks.
The current difficulty can be checked at this link and the estimate of the next re-target here (this estimate is often wildly incorrect just after a difficulty re-target, i.e. when there are only the first few of the 2016 blocks of the current difficulty period that can be used to calculate the estimate).
Historical hash rates and difficulties can be seen in these graphs