This is a very poor substitute for trees. Trees convert gaseous carbon dioxide into solid organic molecules [1].
I am pretty suspicious of the idea that humans (especially the humans who come up with the lowest bids to win these contracts) can store planetary-scale amounts of carbon dioxide over geologic time.
I'd personally much rather see the source of the problem fixed (reduce carbon emissions). I also hope this doesn't turn into an excuse to clear-cut more rain forests.
i.e. We'll cut down the real trees and replace them with efficient carbon collecting "tree" buildings.
It would be very interesting if one of these technologies eventually made carbon capture so cheap that multiple countries could single-handedly afford to reduce carbon levels to whatever they wanted.
At that point carbon levels become an active decision. The words environmental policy would take on a completely different meaning.
I am pretty suspicious of the idea that humans (especially the humans who come up with the lowest bids to win these contracts) can store planetary-scale amounts of carbon dioxide over geologic time.
This may very well accomplish nothing.
[1] http://en.wikipedia.org/wiki/Carbon_fixation