The concept of the Dyson Sphere itself is a fascinating one. I even recall a ST:TNG episode.. "Relics" I think it was, where the crew actually found such an artifact exactly as Dyson had proposed.
If we could design machines that assemble themselves and replicate from the natural resouces(primarily solar energy) available, or maybe harness fusion power, we could have the Dyson Sphere mostly construct itself, gaining in speed as the construct and number of machines involved continues to grow. Ideally, harness the imagined technology of Star Trek, ie. the matter replicator, we could fashion unlimited amounts of material from the energy the Sun already produces.. energy that is almost entirely wasted, except for the infinitesimal amount(relative) that the Earth receives. 😉
As for searching for such a beast.. let's concentrate on finding a signal of some kind, before trying to figure out what type of housing material ET uses. 😛
Edit: As for collision impact, that would depend on the material it's made out of. 😉 Also, consider that if you've got the full power of an entire star to work with, setting up some kind of "forcefield" might well minimize the impace of comets and other debris.
Another point that I didn't see mentioned in the article, but that was mentioned in the Star Trek episode, was that if you were able to construct the sphere at the same life-giving threshhold that the Earth is at, you'd have the surface area of the entire inside of the sphere to work with for your people.. over-population would be moot.
