We came up with our model of tracking independently, but I found out later that Mixpanel has an eerily similar tracking API (see http://mixpanel.com/api/docs/tour). I think it's a fairly natural design, though, and wasn't surprised to see that someone else had thought of it.

Mixpanel does deal with funnels differently, however. With their API, you explicitly define events as being some numbered step in a funnel; in ours, the events are agnostic about what funnels they participate in, and we piece them together in a post-processing step.