See announcement by Scott Guthrie here. In a nutshell, Mobile Services allows you to add an Azure cloud backend to your mobile solution in minutes. Initially, support is for Windows 8, but will be expanded to Windows Phone, iOS and Android. There is built in support for structured storage (SQL), Push Notifications, Authentication (Windows Live ID initially, more to come, read Facebook etc., maybe ACS). The goal is to make it easy for non enterprise developers (i.e. primarily mobile developers) to create a robust cloud back end. I am very excited to dig deeper, expect talks and blogs on this soon!
There is an introduction on Channel 9 here, and a YouTube video here with Scott Guthrie.