Even though it seems plausible that positive feedbacks could essentially push the Earth past the point of any return, it's not entirely the case. Most aspects of the climate system have certain "caps" or maxima. During the Cretaceous, it is believed that the Earth was fairly close to maximum CO2, meaning that the maximum amount of CO2 that could be in the atmosphere was, but the rest was still stored in reservoirs where it was basically immune to feedbacks.

However, if carbon is taken from these stable reservoirs (deep ocean, fossil fuels), this cap is increased. So no, feedbacks cannot cause a runaway greenhouse effect, but humans sure seem like we might be able to.