Most sources I've read suggest that planting even very large numbers of trees would not do much to offset global warming, as mature forests have very little net CO2 sequestration effect. I've also read that the heating caused by the increased albedo of tree leaves offsets even the small gains from this one-time absorption.

Intuitively, it's fairly easy to see why any scheme for removing CO2 from the atmosphere is doomed to failure -- the concentration of CO2 in the atmosphere is fairly low and you're working against entropy to remove it. This leads to very high energy consumption per unit mass of CO2 (far, far higher than the energy extracted from burning the fuel which created the CO2) which pretty much offsets any gains that you obtained by removing the CO2. The key is not to put CO2 there in the first place.

I'd love to be proven wrong about the argument to plant trees, as I'd personally love to see more trees around.