The update will bring resizeable Live Tiles, the Bing daily lock screen wallpaper, twenty new themes, and various small enhancements. The idea is to make those Windows Phone 7.5 users feel like their handset is new and fresh. This might keep current Windows Phone users from jumping to another OS once their current contract expires.
source: Telstra, O2, WindowsPhoneColombia (translated) via WPCentral
All of the signs point to a January 31st start date for the roll out of Windows Phone 7.8