It is Zinc Alloy. Nobody can confirm about the exact density for the keycaps.
Well, unless it's zinc-aluminium or zinc-magnesium, it should still be easily over 5 g*cm^-3 Really, the only other common alloying metals that have densities under 6.5 g*cm^-3 are titanium, vanadium, and beryllium, and I doubt those have been used here due to their expense.
Point is: should be heavy.
Me, I'm going to wait until sets come in, and people review them (look at the "W" in the gold picture) or until I can confirm Dvorak.
---
Edit: it's probably straight-zinc that's been electroplated, according to the post. I like electroplating.