I'm also curious about the speed. Maybe someone who knows more about lens design could comment on it. Is it harder to make a useable pancake with speeds greater then 2.8? I've been reading about micro 4/3rds lately and they all seem to complain about slow prime pancakes.
Well, the point of a pancake lens is to be as small as possible. If you look at Canon lens design (for example, the 35mm lenses), the lower the aperture, the wider the diameter and the more elements. The 35mm f/2 has 7 elements in 5 groups, the 35mm f/1.4 has 11 elements in 9 groups. It's twice the length as a result...which would destroy the pancake design. Seems to go away with the longer focal lengths to a point. Also, Canon doesn't offer any cheap wide angles under f/2.8 anymore.
The other factor is that mirrorless is working with a smaller sensor, so they need even faster primes to begin with, by about 2/3rd's a stop to APS-C. So they'd have to pump out f/1.4 primes to match something like the 50mm f/1,8