There's a very interesting interactve chart on the BBC that draws together three main poling organisations in the US (see graphic) to show how the President has rated over the past 5 years.
The version on the BBC gives clickable links to events in the time that relate to canges in his rating.
http://news.bbc.co.uk/1/hi/in_depth/6038436.stm
In the run-up to the elections you're having, I thought this was enlightening.
What do you all think?