The update to the stopgap version of Microsoft’s mobile OS was approved on January 7 by AT&T, which revealed the release date in an internal email discovered by Windows Phone Central. Users will not be able to receive the update OTA (over-the-air); they will need to side-load the roughly 100MB-sized file by plugging their phone into their PC and using the Zune desktop software. There is no word yet on when the 7.8 update will be hitting other AT&T devices or Windows Phones on other carriers.
Though most of the changes in Windows 7.8 are tweaks and minor upgrades, one of the big features includes the new Start screen, where users will now have a third tile size to help further customize its appearance. There will also be new theme colors available, as well as Marketplace enhancements (including an “optimized download process”), API enhancements, and an improved Lock Screen, which will protect against accidental wipes and can display the Bing “Image of the Day.”
Source: Windows Phone Central