All signs point to tomorrow as a big day for Windows Phone and Mango, so it shouldn't be of too much surprise that those same servers that will be pushing out Mango to the masses have been turned off.
Specifically, some folks are now trying to upgrade to the developer build (7712) or even the security update (7392) for various reasons and they are finding that Microsoft's servers are reporting their phones as being "up to date" even though they were on the very old 7004 (aka pre-NoDo). Users at XDA and WPCentral forums have both reported the same phenomenon of not being able to do any updates.
The speculation is of course that MS is prepping their servers for Mango and therefore have disabled all further updates. We've also noticed the Marketplace has ceased publishing new apps (though old ones are being updated). We're gonna call this a pretty good sign that something big is about to happen and we're not writing it off as coincidence.
Thanks, Ashish Mohta, for the tip!