About the colonizer craze: when should I pack my bags for Mars? Etc.

yes

 

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Nobody would use VASIMR drive to get to another star, especially not at 0.1c.
Only certain types of highly speculative antimatter-powered drive could get a ship to a nearby star in a reasonable time. Alternatively, some form of beamed propulsion could be used- this would remove the requirement for accelerating vast amounts of fuel and propellant with you (except, of course, you would need enough fuel to decelerate at the destination).

One design for a craft that would get to the stars in a semi-reasonable time is the Daedalus concept:
http://en.wikipedia.org/wiki/Project_Daedalus
this used fusion rather than antimatter, and would reach 0.12c on a fuel/payload ratio of about 100/1. Unfortunately this would only allow the craft to reach a nearby star, not slow down; to slow down the ratio would be 10000/1. Large, but not impossibly large.

 

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Of course not. I picked VASIMR because we don't know how to do any better in terms of specific impulse.

I would put this as even more speculative than matter-antimatter.

We don't know how to build a fusion rocket. Scientists and engineers have slightly different concept of what is feasible.

Unfortunately, things don't necessarily scale up that way. Structures are 3D, surfaces are 2D. Look at the kinds of things evolution has done with insects versus elephants. There are some things evolution can do on a small scale that simply do not scale up. The same goes for scientists and engineers. Some things simply do not scale up the way we want them to. Rockets are one of those things.

 

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Yes; a lot of things are speculative, but far less outlandish than faster-than-light warp drive. Here is a beamed power concept by the late Robert Forward,
http://en.wikipedia.org/wiki/Starwisp which interestingly enough, Stross himself used in his SF novel Accelerando.

 

My own image of a starwisp can be found here
http://www.flickr.com/photos/25577499@N07/2410264072/in/set-72157604502272263/

 

Here's Forward's original paper- note the use of a zone plate, a useful bit of kit that might make all sorts of things possible at very long range.
http://www.transorbital.net/Library/D001_AxA.html