You think they'd support Steam on BSD before Steam on Linux? That just seems nuts to me.. they already have an immediate and much larger market for Linux, and in regards to your "other secret sauce" comment - according to
various sources Gabe is interested in keeping an open platform. Custom (forked Linux) kernel like Android??? It's possible, but I highly doubt it as existing games would need to be ported to ARM and it's graphics stack.
Pretty sure they'd rather make their life and the life of the hardware manufacturers easier by releasing on a well maintained and supported platform, for example GNU/Linux.
(edited for clarity)