One new feature in iOS 8.1 is: Text Message Forwarding
Any text messages sent to a phone number on an iPhone will be forwarded to other iOS devices using the same Apple ID.
Go to: Settings > Messages
Enable Test Message Forwarding for other iOS devices. They have to be logged in to iCloud using the same Apple ID with the iPhone.
One thing I noticed is that at this point I cannot forward text messages to other (service-less) iPhones.