My suspicion is that it's going to be a very limited version of the game (map size, civs, and graphics), like the iPad version. Great to allow access on the go, but not the full experience.
And for me, that's probably a net good thing. It gets more people exposed to the game, and allows better mobility for those that are comfortable with it. It stabilizes continued revenue, which should ensure continuous support and success of the game long term.
As long as the game isn't sacrificing for the compatibility (which as far as I can tell isn't so, as Aspyr is simply converting the PC version to other devices, and you can't truly say VI is lacking in mechanics- refinement being another topic), this can only be a net good as far as I am concerned. And converting from iPad to iPhone presumably shouldn't have raised costs too notably.