All are attempting to do very similar things, but they seem to be going about their attempts differently. Urthecast is using the International Space Station as its “camera tripod” for its video camera (s). Skybox is methodically building a constellation of 24 HD video satellites in Low Earth Orbit (LEO). And Planet Labs seems to be big about taking on risk by throwing 28 “dove” (they’re calling this constellation “Flock 1”) imagery satellites in early 2014.
The article also talks about recent history with DigitalGlobe, future products of the three newer imagery companies, and the possible tussle between the intelligence community and public. A worthwhile read for those interested.