hi,
i have a same question here......any one here using ikelite housing that can fit all the staff in one backpack but not case, i saw before one of the dinner keep they thing in one backpack but with nexus housing (a bit campact size)....now i using 2 backpack for my staff.... 
not included diving Equement
I use a Lowepro Vertex AW300 backpack - right at the limit of carryon legality for US flights, over for international. Does fit under the seat even on American's smaller MD80 aircraft.
I pack the Ike housing, spare port extensions, small dome, zoom rings, 2 DS50s, and all ULCS buoyancy arm components in the backpack. With it lenses, 2 camera bodies (neither is "in" the housing), filters, batteries, chargers, and memory cards. Oh, and a small focus light. I slip my dive computer (in a console mount on a quick-release fitting) in the outside mesh pocket.
The large 8" dome assembly and the actual 2-arm tray handle go in the checked bags with the dive gear, the former wrapped in a wetsuit on top of the included neoprene cover.
With the backpack packed like that I cannot comfortably put a 15.4" laptop (standard HP dv6000 chassis) into the backpack as well. It will "fit" if I force it, but at that point the backpack is expanded too thick to fit under a seat and the laptop feels like it's getting strained (flexed) by the mass of stuff beneath it. So the laptop gets carried by the wife instead.
Wasn't challenged with the backpack on a flight to Dominica, including the smaller turboprop leg from SJU. But only traveled with it the once, and that was just before they started charging for checked bags. Will find out in July if I can make it thru again - at least this time the trip is on a 757 so they may be a bit less 'vigilant' solely based on carryon space (vs. based on revenue maximization....)