So I was having trouble getting iMessage to work on an iMac running Mountain Lion 10.8.2.
I sign into the account in preferences and it immediately says inactive.
I can receive iMessages and if I type a response, I can see that one is being typed from the recipient device but if I hit send I get either "account not enabled" or "you are not signed in" errors. Usually the latter. Clearly I am signed in so this is dumb but I cannot send.
I have deleted prefs, tried the account on an iOS device where it works fine and I'm out of ideas. Anyone else?