New Scientist: Imagine Earth without people. Diagram from the article (hosted at TreeHugger.com).

I think about this scenario a lot, for some reason, though with more of an interest in what would happen in the short term. I'd assumed that electricity distribution networks would suffer immediately, and take out most human support infrastructure with it within a couple of days. The article suggests blackouts in 24-48 hours simply from power stations that rely on human effort to load fuel into generators; I was thinking more about cascade failure effects in complex networks. The effect of neglect on nuclear reactors is mentioned, though no timeline is suggested.