Google only gave OnHub a passing mention at today's event before announcing Google Wifi. For some reason, the presenter didn't feel it important to mention that those of you who spent $200 on the OnHub last year are not being left out of all the cool new WiFi stuff. Yes, OnHub will be compatible with Google Wifi.
If you already have an OnHub, you'll be able to add Google Wifi units to your home to build a mesh network. Google Wifi will even plug into your existing OnHub network, so most of the setup is already done. OnHub and Google Wifi will also use the same app, which is currently called "Google On." I suspect there will be some sort of name change. That also means we should see continuing development of both OnHub and the app.
Google's support site says more information will be provided later this year about an update to OnHub that will enable compatibility with Google Wifi. Hopefully this feature will be available around the same time Google Wifi starts shipping.
- Google Support