Hi granthammer,
I put my reg (including computer console,etc.) in my carry-on, along with camera/housing etc., and yes,it does make for a full, and somewhat heavy, carry-on bag.
The reasons: (agreeing with what MikeS said), the reg/computer/etc are the parts of the dive gear that I would
least like to lose,have damaged,or have to rent when I get to my destination.
All the other gear is in a checked bag-no fun if that gets lost,or delayed, but at least I'll know that "the thing that brings me the air" is one that I am familier with, and know is in working order before I need to use it.
Regarding the photo gear in the carryon : I believe that this gear less likely to get damaged in a carry-on,as opposed to the more rough treatment I have seen given to checked bags.
BTW, other folks on the board have said that they manage to get even more in their carry-on: Mask, BC, suit....expert packers, I guess
Good luck,
Mike