Hi there and no worries...
I had the same concerns when I was considering buying a can light earlier this year but they have come to nothing.
Carry your light and battery onto the plane. You will be asked to show the light to security 9 times out of 10: be friendly, smile and chat about what its for. They are usually just curious. Most importantly carry the manual describing what it is and a page of photos of divers using can lights: these pictures really help explain what its for to non-divers.
Note can lights are a fire hazard if placed in check in: some baggage x-ray staff have wired can lights up to test them and then left them wired: if turbulence jogs the switch on you could have a fire (some airlines rule that these lights must be carry on). However I've never been asked to check in the light after its been looked at by the carry on baggage screening.
I've now flown through airports in Mongolia, rural china, Singapore and Australia with the light with no problem. Just smile, show them the manual (in my case a page from Salvos web page) and the pictures and they should wave you on. It helps if you look like you could talk to them all day about it
I have only been asked to turn the light on once (in China).
To facilitate the process I bought a camera-carrying daypack (lowepro) that zips fully open to display the light, regulators and documentation in their compartments: its a lot faster then unloading a normal daypack.
Cheers,
Rohan.