Getting Miracast on older phones & tablets

I'm sure a lot of you, just as I am, are really looking forward to Miracast. Once it's broadly supported imagine how convenient it'll be to be able to "beam" the content on your Android device to any TV, projector or audio equipment!

To get there though they'll be dongles and adaptors for TVs that don't have Miracast built in yet. But what about phones prior to Android 4.2?

This is what I'm trying to work out: Miracast is built on Wi-Fi Direct, it's all software right, a specification of a protocol? And isn't Wi-Fi Direct also software on good old 802.11 (ie. Wi-Fi) to make it a Soft AP? So will Miracast come to older phones and tablets if they get the 4.2 upgrade or an equivalent software stack?? Or is there an essential hardware element?