Well.... Ordinarily Titanium will spontaneously burn on contact with O2 at 47% or higher but it can be made to do so with 35% oxygen under the kinds of pressure you'll find on the HP side of the reg (350psi or higher) *and* with the addition of a catalyst (ie. let's hope your reg isn't "dirty"). With lower concentrations of O2 (under 35%) it won't burn.
At least that's how I understand it. I'm not an expert but I figure if they can get titanium to burn on contact with O2 at 35% in *ANY* kinds of conditions then it doesn't belong in a scuba regulator....... Just as a point of interest, the NASA prohibits the use of titanium in air oxygen partial pressure environments above 30psia because of the risk of spontanious combustion.
R..