I use(d) a Crumpler messenger bag for this purpose, specifically the Seven Million Dollar Home, though you can get smaller ones of course and less garish colours. Holds a 35mm body with grip and medium-sized (24-70/2.8) lens nose-down plus 4-5 other lenses or flashes separated by dividers. Can even fit a 300/4 laying across the top.
Beware carrying a heavy messenger bag for extended periods of time though; the asymmetric load on your shoulders will do your back in.
My 4x5 kit (Toyo 45A and ~10 holders with lenses resting on top) now lives in that bag.