You have to be very carefull with rechargable li-ion batteries. They are considered 3.6 volts. Hot off the charger they are 4.2 volts. You can't just put two rechargable batteries in a light designed for two CR123's. (some lights can handle but not all) There are other options though. For instance, a Surefire L4 or E2L can take one 17670 li-ion battery and run in regulation just fine. Also, you have to have protected Li-ion batteries less they over discharge or over charge. This becomes a big consern as they will vent very violently. Check out www.lighthound.com for all your rechargable needs.
There are some other neat tricks as well. Such as light that run off of 3 CR123'S. You can run them off of two 17500's and it will run as if it where on the three CR123's. I have a 2C cell Maglight that I had converted to run off of 3 CR123's and a 6 cell MagNum Star bulb. (puts out about 120-150 lumens) I then got two 18500's and a charger. It quickly became my favorite light. Then I got an LED module for a 6 cell light and it is just as bright but runs twice as long. :) Lots of options with rechargables but do be careful.