Nate Cohn cautions:
A Reuters/Ipsos poll found Obama leading 48-43, down slightly from 47-41 prior to the debates. The survey was conducted entirely after the first debate, but it's an online poll with a small sample, so don't put too much stock in it.
Nate Silver's two cents:
[T]he volatility of the poll — coupled with the fact that the post-debate interviews consisted of a fairly small sample of about 500 people — means that relatively little should be read into it. Ipsos lists the margin of error on its post-debate sample of likely voters at 5.2 percentage points. And that 5.2 percent figure only reflects the margin of error associated with any one candidate’s numbers; the margin of error on the difference between the candidates is roughly twice that, or plus-or-minus 10 points.