There is another interesting assessment of the limits of high resolution photography at gigapxl.org, including atmospheric effects, which are distinctly worse for terrestrial photography than with a telescope looking up through the thin, clear air from the top of a mountain.
They estimate an angular limit on resolution at 50% MTF due to atmospheric effects to about 6,000 line pairs per radian, or in other words, 6,000 line pairs across a roughly normal field of view of 57º (one radian). To be generous, about 10,000x15,000 pixels for a normal FOV would be at that limit, so maybe 150MP.
From a satellite 100Km up, that resolution limit at 50% MTF would be about one line pair per 14 meters. With the far lower MTF needed to barely resolve text, the graphs suggest a limit of about 20,000 lp/radian, still 5 meters per line pair.
Maybe on a very clear day they can see grapefruit, on the top of a mountain above all the dust and heat haze!