Let me try to explain this from some knowledge of the situation.
Fox does not like to be the first in the pool on any new technology or idea. They waited to do HD news until after many stations had done it for quite some time. Before that they were late adopting non-linear editing and server playback of news content. Generally speaking, Fox likes to let others try and fail then succeed before Fox comes into the market with their own product.
Fox is likely watching to see how things like Antenna TV, Universal Sports, etc do before they commit to any kind of subchannel strategy. It seems likely to me that they will eventually put subchannels on the air. But they won't do so until they're pretty sure they can make money on them. The same goes for mobile DTV services. I know they've already dipped into that a bit, but I suspect there's more money upfront on that than with any kind of normal subchannel.