I read about this theory somewhere a few months ago, but had no real basis for assessing its validity. And just now, via Megan McArdle, I find that it's been written up in the Economist:
The ice-core record shows that the level of carbon dioxide in the atmosphere made an anomalous upturn about 7,000 years ago, and that methane levels, which were also falling, began to increase about 5,000 years ago (see chart). These numbers correspond well with the rise of farming in Europe and Asia.
I can't find the quote now, but I remember reading that someone hypothesized that this human-driven global warming might have staved off an ice age.