This topic came up in my Learning Analytics MOOC and it is something for people to be aware of – a huge sea change in the power of searching is coming once meta-data becomes ubiquitously assigned to content.
The Tim Berners-Lee talk is a great overview and place to start. If you want more details, watch bitly’s chief Hilary Mason – so interesting! Finally, an example of first level implementations of some of this in educational contexts – inBloom example in the edtechdigest article has some good videos about how it works.
Tim Berners-Lee talk
Hilary Mason, Machine Learning: http://www.infoq.com/presentations/Machine-Learning