Agreed. I guess it really boils down to how much you trust the dive op you go out with, and how much stuff you're willing to bring on the dive. The LDSs around here don't carry everything I need, so I head to the outdoor/camping or hardware store to get (or make) what I need.
Anyway, I have a signalling mirror (from REI) that allows you to aim the reflection of the sun at a target. Along with a waterproof whistle, it is attached to my SMB, which I bring on boat dives. I've been told a CD serves this purpose just as well. I've also thought of adding an OMS dye marker for those really offshore or liveaboard dive sites but haven't used one yet.
If one is really afraid of being left at sea and forgotten, I guess other stuff to consider bringing are: inflatable float (e.g., Halcyon Surf Shuttle), strobe/backup light, EPIRB beacon with internal GPS, and the following in a waterproof case: satellite phone or VHF radio, and flare etc.
Dive safely.