I know a couple of pro photographers and they pack up their gear in well-padded Pelican cases that get checked as they often travel with more than they could carry onboard anyway. One includes a starter's pistol in the checked luggage because the FAA classifies it as a firearm and the airlines therefore track it much more closely.
I usually carry my own gear (couple of notebooks, various networking and wireless gear, etc.) in a fairly large backpack, and it usually fits under the seat in front of me. However, as amenities like in-flight power and seat-back entertainment become more common, I'm finding that many of the seats have equipment boxes underneath and getting my backpack to fit is becoming difficult in a few cases.
I have seen two airlines that do size checks: Frontier and Spirit. However, both of them also make more money from checked bags than carry-ons (Spirit charges for carry-ons, too, IIRC), so that may just be a money thing for them. (I haven't flown Spirit, and only rarely fly Frontier, but I frequently pass gates for both.)