I'm not one for woo but I do think there's something to this - only because concepts don't spring forth in a vacuum, no matter how original they seem to the person who came up with them.
Taking the Pripyat example - a quick read over the wikipedia entry shows that around 2005 someone started running guided tours there. Under the 'cultural references' section there are a variety of documentaries, stories, songs, video games, and movies that have used the setting in some way - most of which came about in the last 6-8 years, culminating in a full movie with mainstream distribution based entirely in the city being released this year.
So I imagine it went something like this: after 10-15 years the radiation had dropped to a relatively safe level. The first people to go there were urban-explorer types who were willing to take a risk that the radiation might still be dangerous. They post photos on line of this cool abandoned city, and the photos get spread around virally (boing boing, the world's most popular blog, has featured several of these sets of photos over the past 8 years or so). After a few years it's clear it's relatively safe, and someone who saw some of those photos decides to start a tour business. Now people are going on a regular basis and a steady stream of photos, videos, and stories are being shared online. Suddenly, tens or hundreds of thousands of people around the world who might have never even heard of Pripyat are all becoming aware of it around the same time - it's entered the 'idea-sphere'. Someone developing a video game needs a cool setting and remembers hearing about this place. Someone in a band writes a song about it. Documentary filmmakers seek it out as a subject, and narrative filmmakers use it as an element of their stories. Everyone doing this thinks it's a unique creative idea - except that at the same time thousands of people around the world are all having unique creative ideas sprung from the same seed. Some are more alike than others, most will never come to fruition, but inevitably a few will emerge over time that will seem very similar to one another.
So to me, that's the idea-sphere - not that something actually escapes your consciousness and is floating around to be picked up by others, but that things born in our consciousness tend to come from common points of reference that many people have picked up simultaneously, often even unconsciously.
I think this happens far more often now with the broad availability of the internet than it did in the past. It's common for unique and interesting subjects to go viral online - while still not crossing directly into the mainstream. When this happens it's like thousands of seeds being planted, and it's inevitable that within a few years you'll see similar trees grow from those seeds.
