Philipp Lenssen at Google Blogoscoped stumbled across an interesting, and hidden, feature called Google Tasks. There was language in the FAQ that definitely made it sound like its existence is supposed to still be under wraps.
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There is no proof that this application is definitely going to be released -- in fact, the last time the FAQ was updated was before Gmail released their own task list. I personally don't believe it's going to be released though, as it's function would make sense to belong as part of their other applications rather than by itself.
The application existed at http://www.google.com/tdlist until Philipp's article was posted -- it has now been removed.