(see below link)
And it really hasn't moved much at all from the August date of the Gallup poll you mention among the rest.
National → US-Approval - Obama
Top Matchups
Favorable + Unfavorable
Approve + Disapprove
Poll Results
09/07/11 Gallup permalink Disapprove 50.0% Approve 44.0%
09/07/11 Rasmussen permalink Disapprove 56.0% Approve 43.0%
09/06/11 Gallup permalink Disapprove 50.0% Approve 42.0%
09/06/11 Rasmussen permalink Disapprove 56.0% Approve 42.0%
09/05/11 Zogby permalink Disapprove 61.0% Approve 39.0%
09/05/11 Rasmussen permalink Disapprove 55.0% Approve 43.0%
09/04/11 Gallup permalink Disapprove 50.0% Approve 43.0%
09/04/11 Rasmussen permalink Disapprove 55.0% Approve 44.0%
09/03/11 Gallup permalink Disapprove 50.0% Approve 42.0%
09/03/11 Rasmussen permalink Disapprove 55.0% Approve 44.0%
09/02/11 Rasmussen permalink Disapprove 54.0% Approve 45.0%
09/02/11 Gallup permalink Disapprove 51.0% Approve 42.0%
09/01/11 Politico/GWU permalink Approve 50.0% Disapprove 45.0%
09/01/11 ABC/WaPo permalink Disapprove 53.0% Approve 43.0%
09/01/11 Gallup permalink Disapprove 50.0% Approve 42.0%
09/01/11 Rasmussen permalink Disapprove 55.0% Approve 44.0%
08/31/11 NBC/WSJ permalink Disapprove 51.0% Approve 44.0%
08/31/11 Gallup permalink Disapprove 50.0% Approve 42.0%
08/31/11 Rasmussen permalink Disapprove 56.0% Approve 43.0%
08/30/11 Gallup permalink Disapprove 54.0% Approve 39.0%
08/30/11 Rasmussen permalink Disapprove 57.0% Approve 42.0%
08/29/11 Gallup permalink Disapprove 55.0% Approve 38.0%
08/29/11 Rasmussen permalink Disapprove 56.0% Approve 43.0%
08/28/11 Rasmussen permalink Disapprove 55.0% Approve 45.0%
I didn't place a lot of weight on that 8/29 or 8/30 Gallup when it came out, because it seemed pretty anomalous among all the others running in the 42-44% range around those dates. Which is still pretty much where he is overall. He's been steady in the low 40's over the past few months.
That is a somewhat appreciable decline from late spring, though, when he was still running in the higher 40's to breaking 50's. (see graph)
http://polltracker.talkingpointsmemo.com/contests/us-approval-obama