According to Superhero Hype:
Several people were dangling from cables over the Cape Fear River Friday. The production of Iron Man 3 is expected to film scenes in that area next week and it looks like this stunt could be part of the crew preparing for some special effects.
The footage was captured by WECT of Wilmington, NC and their evening anchor Michelle Li confirmed to Buzzfeed the film has a permit for the area next week, making this look like a practice run.
Cape Fear River travels almost the entire length of North Carolina, which is where Marvel is rumored to be filming, so it seems plausible.