Getting off the planet isn't the problem. I agree with Doug that a dedicated group of engineers could do that quickly--perhaps in a few weeks. The problem is how to survive off-planet when we get there. Biosphere notwithstanding, we have insufficient knowledge of what is necessary to sustain human life. We know there are hundreds of species of bacteria living symbioticly on us, but we haven't even named them all yet. We've had astronauts in orbit for a few months with minimal resupply, and they have few health problems other than some musculo-skeletal degeneration, but nobody knows if humans would be fertile in those circumstances or what pieces of Earth biosphere are needed to eliminate the resupply. Rebuilding a viable population wouldn't take more than a few tons of humans--a dozen young, healthy, diverse fertile women and a dewar full of sperm--but any small lack of foresight or setback that prevented them from bearing healthy fertile children would doom the colony, and we'd need about 16 years before we'd know.
-- Lee Daniel Crocker <firstname.lastname@example.org> <http://www.piclab.com/lcrocker.html>
"All inventions or works of authorship original to me, herein and past,
are placed irrevocably in the public domain, and may be used or modified for any purpose, without permission, attribution, or notification."--LDC