Obama just held a press avail here in San Antonio. He took eight or nine questions, two of which concerned the NAFTA flap. His basic line was that Goolsbee went to chat with the Canadians as a courtesy to them, at their request--this wasn't something the campaign initiated. Then he argued that Goolsbee didn't really say anything different than what he (Obama) has been saying on the campaign trail, which is that he supports trade with Mexico and Canada but wants to make sure the agreement is fair to American workers. He argued that he's had the same position since his 2004 Senate campaign, and that it's Hillary who's being cynical here. According to Obama, she's the one who supported the agreement up until she decided to run for president.
In general, Obama chalked these sorts of questions up to the "kitchen sink" the Clinton campaign promised to throw at him in the homestretch.
I've only been to a couple of Obama press avails since the campaign started, but, for what it's worth, this was unquestionably the most hostile one. In addition to NAFTA-gate, there were a couple of Rezko questions, which the Chicago press corps in particular was keen to tangle with him over.