For August 23, 2011, the results are as follows: strongly approve 19% minus strongly disapprove 45% equals a Presidential Approval Index of -26. Rasmussen says:
This is the lowest Approval Index rating yet measured for President Obama. The previous low was -24 reached yesterday and also in September 2010. Additionally, the level of Strong Approval matches the lowest yet recorded.
Gallup and Rasmussen are showing the same negative trend lines, scroll down for Rasmussen's graph.