The 23.85MB download will bring your device version to 1.28.401.9, and while it doesn't bring visual changes (apart from an auto brightness tweak), it does bring enhancements to supposedly fix the battery issue (according to HTC, it brings 20% more battery life), performance issues, and includes some minor bug fixes.
You might not immediately see a notification for an available update, but you can always manually check for the update in your settings menu if you have the device. I just now manually checked on our device and found the update. To do this, go into settings – scroll to the bottom of the page and press "About“ - press the top button that says "Software updates“ - and then press the button at the bottom of the screen that says "Check now“. (A screenshot of the update notification on our One X can be seen above).
I haven't been able to install the update yet, as I'm getting a message saying that it's downloaded, but can only be installed if your battery is charged above 35%. So while I wait for it to charge up (aaarrrggghhh), let us know in our comments or forums if you receive the update!
We'll report back with any noticable changes or fixes we see from this update.