Well, but isn't the CO2 released from petroleum technically bio-derived as well? (That is, unless you're one of those crackpots who doesn't believe oil is biogenic.) There's no Co2 coming out of my tailpipe that wasn't taken out of the atmosphere by plants millions of years ago. Of course, we'd prefer that fixed carbon to stay in the ground rather than being returned to the air, but it's all biological processes we're talking about, here.