There have been a number of blog posts over the last two days about interstellar travel and the difficulties involved therein. As reported in Wired, Robert Frisbee, group leader in the Advanced Propulsion Technology Group at JPL, conducted a study that designed an interstellar vessel with an antimatter-based propulsion system that could reach α Centauri in a mere 40 years. Brice Cassenti, associate professor in the Department of Engineering and Science at Rensselaer Polytechnic Institute says that it would take between one and 100 times the current energy output of the entire world to send a probe to α Centauri. Many scientists at the recent Joint Propulsion Conference analyzed the proposed designs and largely agreed that traveling to even the nearest stars within the human lifespan is nearly impossible.
Randall Parker at FuturePundit points out that the development of therapies for rejuvenating people will make it possible to live long enough to travel to another solar system, but wonders if anybody would be willing to spend 50 years traveling to reach another star system if all we find there are planets like the ones (other than Earth) in our own solar system.
Paul Gilster, meanwhile, points out that thrust-based systems (ejecting mass backward in order to go forward) are not the only means of propelling a spacecraft, and remains positive in spite of his bet that an interstellar mission will not be launched before December 6, 2025.
Brian Wang agrees that newer technologies may change the basic assumptions Frisbee used and eventually make interstellar travel possible, but points out that advances are needed not just in propulsion but in materials. And people.
There are some very smart people hard at work on solutions to the interstellar travel problem, but I suspect that Paul is right... they won't find a good solution in the next few decades. I do think, though, that we'll solve the problem eventually. I very much hope that I'm around to see it (possibly with the help of the rejuvenation therapies Randall mentioned).
If you want to participate in the ongoing discussions, Paul's Centauri Dreams site serves as the discussion area for the Tau Zero Foundation. Also, The Ultimate Project has forums to discuss their 500-year plan for a massive interstellar colonization ship. I'm sure there are other sources as well, but those are the two that immediately come to mind.