An interesting idea, though one wonders about critical mass and so on. It seems to me that there's a useful question to be asked about when it makes sense to build a vertical application like this (by which I mean, an application targeted at a particular audience) and when it makes more sense to use the tagging facilities in existing social tools to partition various sub-sets of the content.
For example, one presumably wouldn't ever invent Flickr4learners?? Rather, one would use a suitable set of tags to highlight images particularly intended to be used as learning resources (i.e. tags that indicate curriculum area, educational level or whatever).