Issues with iMessage?

Is anyone having difficulty getting iMessage to work correctly?

I wanted to test the unified Apple ID this morning and sent a few "test" messages from my MBA to myself. They appeared on my phone like magic.

I then struck up a conversation with a friend of mine who has an iPhone (iOS 5). His response to a message I typed on my MBA appeared on my phone. Now, any message I attempt to send from the desktop is "undeliverable."

iMessages have also stopped synching across my devices. Any "test" I send from my MBA no longer appears on my phone. I'm at a loss!