As far as I know will not be supported yet! Apple need to release something like a unified apple id... According to some blogs... There's a work around if you change the caller ID to your email... On iMessage settings on your phone. After that you need to restart the conversations (write a new text message from scratch .. To those people that you want this feature)
That works with all my family members... We receive the messages to our phone, macs and iPads ......all are sync, and we all can star writing a message on our phones and continue it on te iPad (for example)
I'm still with iOS 5