There is enough carbon available to do it.
What prevents it is the rock weathering, that is the hotter it gets (or the more acidic the rain gets) the more it rains the quicker rock erosion create calcium carbonate. Specifically the erosion of uplifted calcium beds from the ocean floor.
Also for this reason periods of dramatic orogeny (mountain building) tend to have a carbon poor atmosphere and ice ages (ie now with the Himalayas and Tibetan Plataea).
The weathering create carbon calcium that sequesters CO2 on the ocean floors. Eventually that carbon reaches the edge of a tectonic plate where this happens
http://www.youtube.com/watch?feature=player_detailpage&v=ryrXAGY1dmE#t=34s
But if the rate of release of CO2 from the long term sequestering of the earth such as the northern arctic soils, the ocean clatherates and the CO2 in the ocean were to all be released due to warming then there may be a small chance that the weathering would not work fast enough. As far as I know it is still an open question.
Perhaps the strongest indication comes from the Cryogenian, or snow ball earth, where the planet was near totally encrusted with ice. This meant the planets albedo was very high and it took a huge amount of green house warming to break the deep freeze. This happened because without rain there was little sequestering of the carbon and it built up over tens of millions of years from the volcanoes, eventually enough to begin a melt that likely caused a very rapid flip over in state. This caused a super greenhouse and lots and lots of errosion and sequestering of carbon. When you can find rock old enough it stand out clearly
So it is likely that the super heated climate would create enough acid rain to sequester the carbon rapidly. But its not a climate you would want to be raising children in.
IIRC above an average temperature of 67C for the earth, rain becomes impossible in places so the mechanism slows and IIRC as the troposphere expands deep into what is now the stratosphere, the amount of water vapor at those altitudes increases rapidly increases and the hence there is much more UV (Im sures its UV and not cosmic rays) to disassociate hydrogen from the water vapor and the hydrogen boils off into space. At that point you begin to (very slowly) lose the oceans, when there is more hydrogen being lost to space than gained from comet impacts. Then you begin to in effect boil off the oceans. This is where the Venus effect comes in.
So in summary, theoretically I think it is possible. But I believe there is good grounds for thinking it will not happen yet. Every 100 million years the sun gains about 1% more energy, so the gap to runaway drops. We may only be 500 million years away from it happening.
