That's very, very interesting. Interesting that the Mac Pro isn't recognized, despite it being real Apple hardware and identified as such... I wonder if the iMessage server is "smart" enough to block out legit Apple hardware IDs for which ML is not supported...
I was also wondering why this problem had only suddenly come up recently, and your explanation of a server-side change would explain it very neatly indeed.
Thanks for the post!
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Darius