Update: Google also announced today that T-Mobile will now allow MMS messages to be sent to Google Voice users:
T-Mobile has allowed MMS messages from their users to be sent to Google Voice users. Similar to MMS messages from Sprint users, these messages will appear in Gmail, and you can be notified of the message via SMS. We continue to work with other carriers to broaden support for messaging and bring MMS to Hangouts. More good things are coming!
Last month, before an official Nexus 5 and Android 4.4 KitKat announcement, Google announced during its Google+ event that it would soon be integrating SMS into the Hangouts app. Starting today, all Android users can download the updated app, which in addition to building in SMS/MMS functionality, also brings animated GIFs, location sharing, and new status icons.
Also available today is an update for Google Keyboard that introduces some of the improvements Nexus 5 owners have enjoyed with KitKat so far, but many users seem to be upset (rightfully so) that the update makes emojis only compatible on Android 4.4 devices at the moment:
Both updates should be rolling out through Google Play for your devices shortly.