The Windows phone 7.8 update is scheduled on Early 2013 based on Windows blog post and it will not be launched this year. It is not too long and we can expect few features of Windows phone 8 coming to 7.8.
In the Windows blog, please find the date mentioned in their post
"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."
Below is the video of Lumia 900 running on Windows phone 7.8.