One thing we tried to push at MIX11, despite all the awesome news, was all that info was for developers, not consumers (though we tried to show why it mattered). The point being that Microsoft still has tricks up their sleeves for Mango/7.5 that we have yet to see and we're sure there will be plenty.
Evidently one of those is a built-in, native check-in system analogous to Foursquare or Google's latest addition to Latitude. This will be done via your Live account, which makes sense since you use that to "log onto" the phone.
All of this comes way of one @KADacey, who is testing said system with the Twitter source '(WindowsLive)INT', which when clicked links back to the Windows Phone site (curiously, he has since deleted the above Tweet, ahem). To make it more interesting, Manan Kakkar points out that '(WindowsLive)INT' is the Live Messenger Facebook app. So take that for what it's worth, which we think is a lot.
Big thanks to clubdirthill for finding the original tweet and Manan Kakkar for digging deeper