The news originally surfaced earlier today on Civilization site Apolyton and was later confirmed by Electronic Arts, according to GameSpot. The decision requires Johnson to move from Maryland to California.
There is no indication as to why Johnson left Firaxis or in what facet he is helping with Spore this late in the game's development cycle. (The title, having been in development since 2000, is scheduled for release by the end of this year.) Of course, there's the obvious possibility he'll be helping out with the civilization phase of the game. Johnson's personal blog has not been updated since this year's Game Developers Conference. As Gamasutra notes, Johnson had been working on an original IP for Firaxis; its fate is currently unknown.