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Pretty sure iPad, iPod touch and Mac based Messages must use email. Kinda sucks since most of my conversations are already tied to phone numbers, and it creates a separate conversation on the other persons device when I contact them using my email.
You can send messages FROM your Mac using your registered iMessages email address TO and FROM an external Phone number.
You CANNOT receive messages on your Mac that are sent TO your phone number.
For example... iMessages sent to my iPhone Cell# 630-XXX-XXXX will NOT appear on my mac. There is no way to program your cell# to "receive" on your Mac.
It is a major downfall. Good description (and idea for solution) here: