If your looking for a single cell light that can operate on a single AA or 123 then check out the JetBeam RRT-0
Awesome light with variable brightness controlled by a twist ring.
With the optional extender it allows you to use the single AA. Remove it and you can use a single 123.
I have a bunch of lights and the RRT-0 is my favorite.
wordity word word word
the current RRT-0 is something like *255* lumens; and brightness is controlled by a MAGNETIC rotating switching interface and is "infinitely variable". in a word "dimmer" control. rotate the ring one way, it gets brighter, not in steps, but smoothly. rotate the other way, dimmer. you get exactly the amount of light you need.
they had a run of a model that does 310 lumens in titanium body. slick

needless to say, they sold out fast.
best part is they use rechargeable batteries TOO if you want. no more buying and throwing away CR123s, esp if you use your light a lot.
i know the main vendor i use offers a significant discount if you are on the right fourms (USN, CPF, etc) and know the magic word. i should ask him if he wants to swing over here, and offer us something something

"BeckerLight" or something
the M1X is the end all be all MagLite replacement. almost nothing on the market compares. throw some rechargeable batteries in that, equiv to 4x123 and boo yah. we won't even talk about the RRT-3

and duh, they have weapons mounts. "who's there?" "MY EYES MY EYES I MEAN BRAINS"