Gav,
Reading Lester Browns article in more detail what he is proposing could be implemented by 2050, but it seems totally unrealistic to replace ALL oil,ALL coal and 75% of NG with wind and solar in 12 years!.
In Australia's situation its clear that we can only make significant reductions in carbon emissions in 12 years by phasing out coal. It seems that the Garnaut Report is taking aim at eliminating coal, with carbon sequestration the sugar an a bitter pill. There is no way that any significant CCS will occur in next 12 years, but could see existing coal fired plants converted to CSM.
Taxes on "other carbon sources" are really included for fairness and increases in petrol prices are going to be collateral damage. The world oil price rises are going to do much more to reduce consumption than any CO2 tax.
Direct legislation to directly reduce coal use(for example mandating 1% NG replacement and 2%wind or solar replacement of coal use per year; much larger capacity changes) and improve vehicle fuel efficiency by 50% could allow CO2 targets to be met with a very mode CO2 tax(much less than required to make CCS viable).
So by 2020 we would have 20% of electricity generated by solar and wind, 6-8% from hydro and a big part of the remaining coal generation capacity able to progressively switch to NG to continue to meet targets beyond 2020. So instead of coal being used as the cheapest base-load, it could be used during longer periods of high demand(heat waves,low wind periods), or low hydro storage(droughts) or as is occurring in WA if there was an interruption of NG supply. Wind and solar would eventually be the low priced fuels, a mix of NG/coal to top-up predictable changes in demand and hydro capacity mainly used for short term grid balancing(greatest value hence highest priced).