Brian Dears has a very nice post on "the long trail", the old is new kind of phenomenon that we've all been feeling lately.
The thing about remixing is that it makes something new out of something old. The act of remixing itself is nothing new. Remixing even in media is nothing new. Remixing in music in particular. Maybe Larry Lessig only found out about remixes in the past few years, but mash-ups, remixes, all that stuff has been around for decades. The path hewn by the pioneers of remixing goes way back. It's a long trail...
So much of what we think is new is not. In computing technology, this is painfully true. Particularly if you were around and actually used the old technology (say, the PLATO system). Speaking of PLATO . . . seeing Ray Ozzie now reporting to Bill Gates . . . now there is a long trail story if there ever was one. Ray started out on PLATO in the 1970s, went on to build Lotus Symphony and then Lotus Notes, then built Groove . . . now absorbed by Redmond.
Jon Udell posting on Screencasting, as if it is the newest cool thing . . . old hat in the computer-based training world. A world, by the way, that has gone through many a relabel over the past 40 years: programmed instruction > computer-assisted instruction > computer-based education > computer-based learning > web-based training > e-learning. Same old thing, really.
From one of the comments on the article which I think it puts it very well. This is the real difference between RSS and BackWeb, between tripod and live journal;
But I think each new emergence gets filtered through the current environment, leading to those slight differences in degree, that (may) lead to differences in kind, as Jon Udell suggests. I think you made a key distinction between a "significant" number of users and a "substantial" number. I would say that "substantial" is the critical mass needed for this "new" technology to become self-sustaining. Each emergence of an old technology or idea into a new context affords the chance that a critical addition or modification can be made, which then attracts more attention, leading to more users, and so on...
I've been looking too literally at comparisons and I think that this comment hits it right on the mark.