Dunno if anyone's seen this article, but I found it quite interesting. Basically it's a summary of field testing done with Biochar in Cameroon showing that biochar effectively increases crop yield for maize by the same amount as either organic or mineral fertilizers alone. Of course adding said fertilizers to the biochar increased yield even more...
Biochar Fund - fighting hunger, deforestation, energy insecurity and climate change - Biochar works: Cameroon trial data show strong improvement in maize yield