Everyone else has been linking to this, so why not: Worlds Beyond Our Own: A Discussion of President Bush's New Vision for Space Exploration

I haven't made it all the way through yet, but here's a good distillation of the "inspiration" argument in favor of manned space exploration:

Now, there's another issue, though. There are issues that go beyond the issue of scientific return per dollar. And this is -- has to do with values; it has to do with challenge, if you wish. Put simply, a humans-to-Mars program would be a challenge to our society and, most particularly, to the most important part of our society, which is the youth. It would be an invitation to adventure to every young person in this country: learn your science and you can become part of pioneering a new world.

That is the challenge that the youth of my day, the 1960s, got from the Apollo program. As a result of that challenge, I'm an engineer. As a result of that challenge, the number of scientists and engineering graduates in this country doubled in that period, at every level: high school, college, Ph.D.

And what did those people end up doing? Well, a couple of them ended up doing what I'm doing, doing aerospace. Most of them went off and they did other scientific activities; they engaged in medical research, they built Silicon Valley, they created the economic boom of the 1990s. Those 40-year old techno-nerd billionaires of the 1990s are the 12-year old boy scientists of the 1960s. That's the investment.

And it is a question of values, of societal values, conveyed to youth. The chance to do something -- yes, heroic. To advance humanity on the frontier of humanity. This is a question of basic values of society.