Tough to power competitively with lithium ion rechargeables the last time I checked, though. Back in the day we'd use 2x17500 or 2x18500 LiIon for more watt-hours than a single 18650, but the revolution in power density for the 18650 was never applied to those other cells.
Unless things have changed, it made more sense to put in a modern 16650 and a spacer, essentially making it into merely a rechargeable Z2 that's 50% longer.
Which is unfortunate; I have always liked the 3-cell Surefires. :-/
Years ago I gave up on powering my SF C3 with LiIon rechargeables; now I merely load it with 3xCR123 primaries for backup, to my superior 'two-cell' 1x18650 lights.
Edit: Well, there is still the 2xAA implementation, but that's just for people one cell shy of a full load. ;-)