Interesting. I think the real challenge behind this though, is not the API itself, but that for it to be really useful there has to be some sort of semi-automated form of data collection, that can be generally applied.
this is why i believe a smart scale is one of the best examples of good data tracking: you step on it whenever you think of it (not on some weird schedule; nor do you have to wear it or take it with you). it does the mundane stuff of capturing the data and sending it to the cloud, which you would otherwise have to write into some notebook.
if it's a system that's fully automated though, one might lose interest in collecting it in the first place: the QSers always like to say – when you add a little friction to a process, you end up paying more attention to why it is. so instead of just blinding capturing your weight, setting up alerts for goals and seeing trends fall and rise will get you to make the most of the data – instead of just collecting data for data's sake.
Ooh, we have a solution to that problem (losing interest in collecting it if it's fully automated). Assuming you're collecting data that you want to change, like, for many people, their weight, or, say, how much time you spend on Hacker News: http://beeminder.com/d/hn
Namely, set up a commitment device that forces you to keep your datapoints on a path to some goal. That's what Beeminder does.
Relevant: http://blog.stephenwolfram.com/2012/03/the-personal-analytic...