Microsoft explains the mobile Internet improvements in Windows 8 over at the Building Windows 8 blog.
We knew that if we were to give you true mobility, that Wi-Fi alone would not be enough. Therefore, for Windows 8, we fully developed and integrated mobile broadband (MB) as a first-class connectivity experience within Windows – right alongside Wi-Fi.
We first included mobile broadband in Windows 7, but if you were a mobile broadband user, you likely had a number of hurdles to overcome before connecting with mobile broadband. Yes, you needed the requisite mobile broadband hardware (e.g., mobile broadband dongle or embedded module and SIM) and data plan, but you also needed to locate and install third-party device drivers, and in some cases software, before ever getting your first connection. If the drivers for your device and software from your mobile operator were not available locally, you had to find another connection type (perhaps Wi-Fi) to the Internet to search for software on the websites of the PC maker or mobile operator. This placed a sizable hurdle in front of users trying to connect with mobile broadband, right when they most needed that connection.