Re: R123\'s - what\'s the latest?
Normal lithium 123 primary cell - 3.0 volts. The lithium ion rechargeable R123 delivers 3.6V nominal, and as much as 4.2V when it is coming fresh off the charger. They are the same size as a normal 123.
If you pop the thing into a 6V light (I'll use a Surefire 6P as an example here) which normally takes 2 x 123, you will get voltages as high as 8.4V, which will burn the lamp out in VERY short order. If possible, you should substitute the lamp with a 9V unit (SF P90/91) and you can then enjoy more light, with less guilt (rechargeable /ubbthreads/images/graemlins/grin.gif). So don't put them into 6V lights, bear in mind the different voltages.
The idea is to use components which can tolerate the higher voltage. One configuration I plan to test is the E1e+KL1, which is a one-cell config, but the KL1 is spec'ed for voltages from 3V to 6V, so feeding it 4.2V instead of 3.6V would be fine. Another config is the E1e+KL4 (TW4) spec'ed for 6V but capable of running on as little as 3V (3-6V). 4.2V to that config should be all right as well.
Someone has proposed a 3-cell config (Surefire 3-cell light) delivering up to 3x4.2V = 12.6V, and using a 12-volt lamp in there instead of the normal 9V lamp. That should be awesome to see.
Basically, make sure your equipment can tolerate the higher voltage.
As the cells are not yet available, most of this is speculation and should be taken as such.. but it's part of the fun of being on the cutting edge. I'm in line to have my P90/91 and KL1 blown up (though I'm quite sure the KL1 won't blow up because it's rated for so much more).