I did end up making a mini XP-G keychain light. However, due to lack of time and interest I only got as far as coupling 2 100mAh LiPo cells in series too two current limiting IC’s for a total output of 700mA. This was all mounted on a strip of vero board along with a switch and a small L shaped strip of aluminium for heatsinking/mount of the XP-G.
As you can see its not pretty, but it gets the job done. It could be half the length it is now with the use of a different switch and by moving the constant current IC’s to a new position. Run time is estimated to be around the hour mark.. but with no cell protection or monitoring I havent played with it for me than 10min max. Even then its in short bursts to keep the heat down.
If I was going to do this properly, it would need a metal shell for heat sinking, an under/over voltage protection circuitry (possibly something from a Lithium Iron battery protection IC as sold on dealextreme) and a way to charge the cells.
Unfortunately I have no way of charging the cells at this stage, so with no more time or interest, its going in the draw for now. On to the next project, building a Reprap Mendel.