The weight limit is for the carry on luggage but usually excludes a "small" backpack and also excludes a laptop in a sleeve case and "reading material". It also excludes anything on your person. I exploit all of these areas. I use a roller case to hold bulky items. I put heavy items in a LowePro AW400 camera bag (my backpack), have a netbook in its neoprene sleeve with handles, and have a shopping style bag filled with whatever else I want for the flight. My clothes are in the checked case.
When I have more to transport I wear a Domke Photographer's Jacket. It has large pockets and I can fill them (pro DSLR, flash, 4-5 lenses, batteries) and it is not obvious as would be true with a photo vest. The jacket with the camera gear is easily slipped into any nook or cranny in the overhead compartment as it can conform to the available space, unlike a hard case or bag.
It varies also by airline as neither United or American have the 7kg limitation on the carry on piece of luggage. If you have special status with one of their partner airlines you can use that to get an earlier boarding time and have more options for stowing your gear - stay away from bulkhead seats at all costs.
