What United Airlines (for example) states is here: http://www.united.com/page/article/0,6722,1032,00.html

Nothing special for a camera bag in addition to the "one bag and one personal item". Maximum dimensions are listed for the carryon bag, but not the personal item.

Of course, it is entirely possible that airline employees can be lax in enforcing the stated rules (perhaps intentionally giving people slack if it does not look like the carryon space will be short on the flight), but could decide to be strict about the rules on any given flight. (And United does state that they could require checking a carryon if it cannot be safely stowed during the flight.)