The title is misleading. You could already use subscription billing using the old (v2) inapp billing API[0] for quite a while. They added that feature to the new API as well.
Apologies for this being old. I didn't know subscriptions were also in v2 of the billing API. The page sounded a bit as though it might have been a soft launch.
I personally would rather just roll out my own (simple) subscription system using Paypal or something, because the 30% cut that Google takes on Play Store purchases is a bit too much.
Generally, you are not allowed to use other payment methods except it is physical goods/services or the digital goods can be consumed outside of the app.
You can do what you want if not using Play's IAP or subscriptions. Google's trying to avoid "click OK to subscribe for $10/month or subscribe on our website for $8/month"
[0]: http://developer.android.com/google/play/billing/v2/billing_...