UPDATED! - 4 Nov 2012
For those that had issues with Intel WiDi - here's the FINAL ANSWER for Windows 8.
Intel has recently posted their Windows 8 WiDi info and drivers since launch:
Intel Wireless Display (WiDi) in general requires the Intel HD Graphics 15.28, Intel PROSet/Wireless 15.5 for your chipset plus the Intel My WiFi Technology 15.5 (a.k.a. MWT, or My WiFi Dashboard 15.5), plus the latest Intel Wireless Display 3.5 software. See the Intel WiDi System Requirements - all software can be obtained from the Intel Download Center.
Note: Certain versions go together as well, e.g. WiFi 15.1.1 and WiDi 3.0.13 on Win8 Preview, or the latest versions as listed on Intel Centrino WiFi Products (new) with Intel WiDi 3.5.40+ (OEM provided i.e. HP, Dell, Toshiba, etc., or latest version from Intel, in Latest Downloads section here).
Finally, there are Intel Win8 drivers for Intel PROSet/Wireless for WiFi, Bluetooth + High-Speed, and Intel WiFi/WiMAX; check the related download pages to determine which driver supports your Intel WiFi card. Also, some notebooks may require the Intel Centrino OEM drivers in order to work properly. The Intel Driver Update Utility can also be used to automatically detect and install wireless drivers and software.
More info can be obtained on the Intel Wireless Products and Windows 8 page or the Intel Wireless Display page at http://www.intel.com/go/wirelessdisplay
UPDATE: The latest updates from Intel employee rradi: Enabling Wireless Display (WiDi) on Windows 8 RTM (also see Comments pages).
Also note that Intel HD Graphics driver is now 15.28.8 (220.127.116.11.2875).