We should also mention that messages are pushed to all devices you're logged into, so you might want to get used to those push notifications if you carry around an iPad and an iPhone.
In the meantime, Android users will have access to a compatible app in Google Play beginning tomorrow -- just in time to start using those freshly unboxed smartwatches. iPhone, iPad and iPod touch users will be able to display notifications and control music in the meantime, though advanced features like watchface and OS updates won't be possible until the app is ready for download.