The latest update for iOS 11 was not slated to release until Tuesday. However, due to a bug that was crashing devices. The flaw involved third-party apps that used recurring notifications. Since iOS 11.2 addresses that bug, Apple decided to release it on Saturday rather than waiting until Tuesday.
As a result, some US iPhone users now have access to Apple Pay Cash. The feature is an alternative to peer-to-peer payment systems like Square Cash and Venmo. The update will allow users to send each other money right through the iOS Messages app. Payments received are put into an Apple Pay Cash virtual card that resides in the Wallet app. Funds on the card can be used to make purchases through Apple Pay or can be transferred to a bank account.
"Now you can use Apple Pay to pay and get paid right in Messages, or by asking Siri."
The feature was supposed to have rolled out with the release of iOS 11, but it was not quite ready for primetime. Apple did not mention what caused the delay, but since it relies on dedicated servers, it might have been waiting for infrastructure development.
Apple Pay Cash is not the only new feature being added with iOS 11.2. The latest update also adds fast Qi charging (7.5W) for iPhone 8 and X models and "live wallpapers" for the X. Of course, various security patches and bug fixes including the crash bug are included as well.
The update went live Saturday but, the Verge reports that Apple Pay Cash is on a delayed rollout (probably to avoid server congestion). If you don't see it showing up, just be patient --- it is coming. Go to Apple's Pay Cash support page to learn how to get it activated.