From what I was told....
Nickel or kanthal
It's in R&D right now. But between TPC (where they will show it next week) and Chinese New Year holiday (2 weeks) it still might be some time till it's released. Hopefully they can beat the clock and get a few shipments out before then.
Over on the Varitube SX350 thread, OneDesign (who's been a long time YiHi chip vendor) is saying that based on his experience, he's not expecting to see the chip available for sale until March.