If the sorts of skills you want to learn are all very practical and don't have a very high learning curve (Ruby, OpenID, and REST, for example) and you don't need to understand them intimately, then I'm sure this approach works well. Not everything falls into that category: if you want to understand machine learning, or computational biology, or really understand database systems (hardly any of the "Web 2.0" folks do), then this sort of JIT approach to learning new things is probably insufficient.