The limiting factor in coal production is the CO2 absorption capacity of the atmosphere. I want to remind everyone here that NASA climatologist James Hansen has calculated in:

Implications of "peak oil" for atmospheric CO2 and climate
http://arxiv.org/abs/0704.2782

that - if we burn all oil and gas - we must have phased out all coal - where CO2 sequestration is not done - by 2050. This is the condition to keep CO2 concentrations under 450 ppm CO2 and hence further warming under 1 degree. Remember that in the last interglacial periods temperatures were around 1 degree higher and sea levels 5-6 m higher.

Hansen writes in his latest press release (29/5/07)

We conclude that the "dangerous" level of atmospheric CO2 is at most 450 ppm, likely less than that, perhaps substantially less.

"Dangerous human-made interference with climate: a GISS modelE study"
http://www.atmos-chem-phys.net/7/2287/2007/acp-7-2287-2007.pdf

The paper documenting the climate model employed in that study is available now at:
http://pubs.giss.nasa.gov/docs/notyet/inpress_Hansen_etal_1.pdf