Spreading into space
Martin Rees, Stephen Hawking and others maintain(ed) that our only chance of long term survival is to spread into space. (see e.g., Dr. Hawking's talk at TED 2008). Their concern is about the survival in this century rather than billions of years.
This page intends to contribute to a discussion on spreading into space and contents that it is very difficult, maybe impossible, and if it is possible it would likely be done at the expense of others. It certainly seems worthwhile to work on alternatives.
Challenges of spreading into space
The other planets in our solar system and their moons seem a lot less habitable than earth and it might never be feasible to sustain civilizations on them. With the large number of stars in the galaxy, it seems probable that many planets exist that could sustain life. Getting there, however, requires interstellar space travel, which is beyond current capabilities and may indeed be impossible. Wikipedia's entry on the subject describes the challenges well.
Accelerating one ton to one tenth of the speed of light requires at least 125 billion kWh, not accounting for losses. At current prices this energy costs about US-$ 10 billion per ton of a spaceship. Deceleration will require the same again. For comparison: a space shuttle orbiter weighs about 70 tons (the Space Shuttle lift-off weight exceeds 2,000 tons, most of it fuel) and the Space Shuttle remains in relatively low orbit. A vehicle to support a viable group of colonists for many decades of travel and dependably slow them down again would have to be much larger. If it is possible at all, it certainly requires a huge effort to bring even a tiny delegation of human descendants to other inhabitable planets. Worse than money, useful energy utilized for space travel can no longer be utilized for the processing of information, and thus consciousness, and thus happiness, so it is a terrible waste of resources.
Nature might allow shortcuts that have not been discovered yet. It seems that they have not been used by extraterrestrial civilizations to visit us, which makes their existence less likely. So Fermi paradox might have a solution in a cost/benefit analysis i.e., the notion that intelligent beings would rather use energy for the experience of existence than for interstellar travel.
Sending just molecules or machines might be more feasible than human colonists, but any form of activity arriving from earth at such places would compete with what could develop there otherwise. Without a clear notion on why something sent from earth would develop more usefully than what could otherwise exist there such attempts would seem as unjustifiable acts of vandalism.
Issues of spreading into space
If feasible, jumping solar systems and consuming planets would be a longer phenomenon than just staying on earth until we've used it up or destroyed it. If those making the jump went on like we did, however, livable places in the galaxy could be expected to be consumed essentially as quickly as people can travel (in the order of millions of years, not billions) and such consumption would be at the expense of beings that could develop there otherwise.
Unless, that is, some civilization, somewhere found and implemented ways to continue civilizations while conditions favorable to life exist. This would offer a longer perspective and would postpone a miserable ending for those left behind almost indefinitely. If we started here on earth, it would also leave much more time to make a jump feasible.