One aspect you guys haven't covered is the time required for an artificial ecosystem to spread across a planet and acheive global climate change of fundamental order.

Based on earth's history, it would take millions to hundreds of millions of years, although perceptible change might be possible in only 10,000 to 100,000 years with hugely intensive industries spewing out biological materials and greenhouse gasses.

Either way, it's beyond our current focus timescale and presupposes a more patient and determined human race than our current bored, restless, sound-bite society.

For those who haven't found it yet, Martyn Fogg's terraforming website is a good first step...