For those in the UK, you'll be happy to see this on the Windows Phone site. While we expect some phones to not get the update right away, it sure looks like Microsoft is expecting a big roll out over the next few days for nearly everyone there.
So get ready!
Update: Evidently the whole world is now getting this message. We're okay with making that mistake.
Source: Windows Phone UK; Thanks, Little_Jumper and Jan, for the tips!