dyson spheres

The lack of Von Neumen devices can perhaps he explained as Sagan did: "perhaps civilizations would realize the risk of building such devices, and not do so" (paraphrasing). But I don't understand why we haven't found D spheres or large-scale orbital structures around stars (e.g., Ringworlds)...

I've got a question for all the astronomers out there (professional or amateur). It seem like recently our discovery of extra solar planets has been increasing at an ever increasing rate. At our current level of technology and with our current understanding of techniques used to locate extra...

... could construct a dyson sphere? and how long could it take?