The carryon regs often state a "linear size" which is the sum of the width, height, and depth. But that's not the full regs. There's a limit on each of these (e.g. an extra-fat bag might meet the linear size requirement but mightn't fit under a seat or in an overhead compartment). Thus the test cages at the airport. You might be able to find the size of the cage via the airline's website if you look hard enough. Also the limits are sometimes explicitly different for business/cattle class.
Which airline is it?
Gary suggested carrying the camera outside the bag, and as I commented I would be prepared to carry the camera+lens outside the bag. But if it fits in the bag within the size/weight limits you might try carrying it on in the bag and if someone decides it's a problem just take the camera/lens out, sling it over your shoulder and get them to check the bag size/weight again.
When I fly I'm typically wearing my Domke photo vest in the airport: it has lots of pockets for tucking lenses/flashes/cameras/etc into. When I get on the plane I take it off and tuck it into an overhead bin with my bag.
Exactly how you deal with it will depend on what other gear you're carrying, whether you're travelling with friends/family who are prepared to carry some bits for you, the particular airline requirements, the gate staff, etc.
Get there early (not only does it help to cope with any delays, but getting on the plane
ASAP can help you tuck your gear away with minimum fuss) and be cheerful (e.g. don't look worried or you might attract unnecessary attention).