Hall Leute, hier mal der offizielle Changlog von Microsoft:

Update 7.10.8858.136

Also known as Windows Phone 7.8*.

Start. Provides resizable small, medium, and large Live Tiles so you can customize your Start experience in new ways.

Accent colors. Expands the number of available accent colors to 20.

Lock screen. Improves the lock screen experience with accidental wipe protection features and the Bing image of the day wallpaper.

Marketplace and Xbox. Expands Windows Phone Marketplace and Xbox support to new countries and regions.

Fonts. Enhances the Chinese font and improves the appearance of Arabic and other languages.

Other improvements. Includes many other improvements to Windows Phone.

This update is not available in all markets or for all phones. To learn more about what's included in this update, see Windows Phone 7.8.

*Windows Phone 7.8 is the official product name we use to describe this release. However, due to a quirk of software development, the actual OS version is identified as 7.10.8858.136. You'll see both of these names on your phone in Settings Settings icon > About > More info.
WIe zu erwarten war gibt es keine sonderlich großen Überraschungen und soweit ales schon bekannt.. leider..

Quelle: windowsphone.com