You can check for the update on your HTC 8X by heading over to Settings > Phone Update. It will update your device to software version 1532.20.20004.531 from 1532.20.10006.531 and the OS version will be updated to 8.0.10211.204 from 8.09903.10.
According to the change-log for the Portico update, there are improvements to messaging, text-replies to reject calls, web-browsing improvements and better Wi-Fi connectivity. The update is also expected to fix the random rebooting issue and brings the ability to maintain the Wi-Fi connection while the device is in sleep mode.
There is now news when the update will be made available for the Nokia Lumia 810, which is the other Windows Phone 8 devices on T-Mobile’s network. Nor is there an indication on when the update will be made available to Windows Phone 8 devices on other networks.
source: T-Mobile USA | WPCentral