Most notably, the update enables the Shield's "Console Mode," which allows players to link a Bluetooth controller to the Shield to play games on the nearest television, much like traditional gaming machines. Nvidia would obviously prefer you use the official Shield controller to play games in Console Mode, but given the ubiquity of Bluetooth it should now be possible to connect a wide range of controllers to the Shield (including Sony's DualShock 3).
Alongside Console Mode, Nvidia also improved Android support in the Shield. "Hundreds" of Android games are now playable on Nvidia's machine, and the company even went to the trouble of designing a utility dubbed "Gamepad Mapper" which allows for use of traditional controllers in Android games originally designed for a touchscreen interface.
Streaming games from a PC to the Shield has also been improved, with "50 of the best PC games" now supported. To further tempt users, Nvidia has also unveiled a promotion in which those who purchase "select GeForce GTX graphics cards" alongside a Shield will receive a $100 discount on the computer/console hybrid as well as free PC copies of Assassin's Creed IV Black Flag, Batman: Arkham Origins and Splinter Cell: Blacklist.
Full details on this promotion and this morning's update are available on the Shield website.