I noticed this on Hipmunk - I visited the site, and saw "Continue as {my name}." I clicked it and was signed in instantly. Wrote down on my task list to figure out how Hipmunk was doing that, as it was a really neat user experience. It seems this is the answer (and they're even using Hipmunk in the demo screenshots).
Would be shocked if this doesn't become very popular on websites.
Would be shocked if this doesn't become very popular on websites.