While there has been tremendous speculation that the Windows Phone 7.8 update would be released today, it's not going to happen. According to Terry Myerson, Microsoft's Corporate Vice President for Windows Phone, look for the update in early 2013.
The Windows Phone 7.8 update will bring the current crop of Windows Phone 7.5 devices (Samsung Focus 2, HTC Titan II, Nokia Lumia 900, etc.) as close to Windows Phone 8 as possible. Hardware restrictions prevent our old Windows Phones from making the full jump.
"We know you’re eager to get the Windows Phone 7.8 update, and we want you to know that we’re working closely with our hardware and carrier partners to get it tested, approved, and rolled out to as many devices as possible in early 2013. As we work to quickly get this in the hands of our loyal users, we’re also striving to deliver a high-quality release and ensure a smooth transition for our widely expanded services."
The 7.8 update will bring the new Start Screen, Lock Screen, accent colors and more to the Windows Phone 7.5 devices, giving them a new feel.
Source: Windows Phone Blog; Thanks, Julian, for the tip!