I've been having this problem recently with my X10 Mini (E10i the old one)
It first started when I left it to charge in the car for like 1 hour, it was somehow heated up, and when I plugged it off and tried to make a call - it turned off... Didn't minded it then, thought it was some kind of "defense mechanism"
But after that it started randomly to turns off when making call (with MiniCM7) and when taking photos with the Flash ON on low battery.
I reinstalled it to the stock 2.1 rom and I still get the turn off when taking pics with low battery.........
Anyone have any idea?
EDIT: Seems like it continues to happen all the time when taking pictures with the FLASH ON