I looked at these numbers earlier today before the Fox and NYT/CBS polls came out. And the trend was there, though this evening's polls have pushed him a bit further down.
A key point to note is that there's a huge variance between the NYT/CBS and Fox polls. NYT shows a net -12 favorability while Fox shows +6. So something is weird between these two polls. But the trend predates them.
You'll note that President Obama's net favorability has only gone into net negative territory for one brief period during his presidency -- during the height of the debt ceiling crisis.