Fascinating, extremely fascinating, breaking news today: Middle Stone Age sites of Mozambique, dated up to 105,000 years ago, has been found that tools retained abundant remains of sorghum, which was harvested and processed into palatable food (bread, porridge or maybe just beer).
Julio Mercader. Mozambican Grass Seed Consumption During the Middle Stone Age. Science, 2009. (Paywall but the supplementary material is freely accessible and an important read for those who speculate on possible contamination, which seems to have been very carefully ruled out).
News articles on the discovery can be found at Yahoo News, and at Science Daily. According to the latter, besides sorghum, other relevant plant remains have been identified: wine palm, false banana, pigeon peas and the "African potato", a medicinal plant.
However this does not imply farming but it's a clear precursor that was until now believed to have evolved only much later, in the Mesolithic.
I found this highly significative news at a discussion at Music 000001, thanks to Glen.
Update (Mar 9 2013): the paper is available at Julio Mercader's profile in Academia.edu → DIRECT LINK. He also has some other very interesting materials on that area near Lake Malawi and in general on African Prehistory.