Clouds of volcanic ash reflect sunlight and cool the earth. The latest climate data from Europe show pronounced cooling for the three years after a major volcanic eruption. The figure shows the average cooling effect of all the major eruptions since 934 (22 eruptions total), from Guiot et al. 2009. The darkest blue is equivalent to a cooling of 2 degrees celsius.
So Eyjafjallajokull may balance out a few year's worth of CO2 emissions.