From this terraforming paper by Martyn J. Fogg:

http://www.users.globalnet.co.uk/~mfogg/paper1.htm

The first martian terraforming models to be published in the technical literature were by Burns and Harwit (1973) and Sagan (1973). They were based on the now defunct "Long Winter Model" of the martian climate (Sagan et al. 1973) which held that up to 1 bar-equivalent of CO2 ice was sequestered in the polar caps, the release of which was driven by the insolation changes caused by the 50,000 year precession cycle of the planet's equinoxes. Sagan (1973) speculated that the caps might be evaporated in

just ~100 years by artificially reducing their albedo, causing them to absorb more sunlight. A subsequent NASA study (Averner and MacElroy, 1976) suggested this darkening might only have to be quite subtle to initiate runaway conditions–a reduction in polar cap albedo by just a few percent, from 0.77 to 0.73. Blanketing the polar ices with layers of dust, or by the growth of psychrophilic plants were variously suggested as ways of effecting this darkening (Sagan, 1973). However, although the mass of dust indicated by Sagan's calculations did not appear prohibitive, the stability of any thin dust layer in the face of the martian winds is open to question. As for the plants: no known photoautotrophs are capable of survival and growth anywhere on the surface of Mars.

Could we genetically engineer plants that could survive and grow on Mars to do what Sagan initially planned?

An interesting Mars terraforming calendar:

http://www.geocities.com/fra_nl/

"There is nothing more difficult to take in hand, more perilous to conduct, or more uncertain in its success than to take the lead in the introduction of a new order of things."

-Jean-Jacques Rousseau, 1712-78