Version 3.1.0 U available via wireless Internet connection includes the following improvements:
Improvements to Standby Functions:
The standby functions will now regularly connect to the Internet when the Wii U is powered down to check for available SpotPass data or software and system updates. Software updates will be installed while the Wii U is powered down, and system updates will be installed the next time the Wii U is powered on. For further detailed information, click here.
Improvements to system stability and usability:
Further improvements to overall system stability and other minor adjustments have been made to enhance the user experience.
*A system update providing further additions to system functionality is planned for release between the end of September and beginning of October.
Version 3.0.1 U available via an Internet connection includes the following improvements:
Further improvements to overall system stability and software compatibility
Source: Nintendo of America