Wait...I think I figured it out. When you click Receive At, click Apple ID and Sign Out. Once you've signed out, the Receive At turns to "Use your Apple ID for iMessage." I think that fixes things!
Turn iMessenger off then back on, works
So here's the über answer for all this nonsense that is Apple iMessage
When you sign into iMessage (on an iphone) using your apple ID. It does 2 things.
It not only registers your phone number as an iMessage device but also the email address as well.
So under Settings>Messages>Receive At
You should have your phone # AND your Apple ID email. These are all of the address (so far) that iMessage will use as your device address (so to speak). So if your friend, neighbor, boyfriend, cousin, baby-mama or whoever uses an iDevice to send a message to this address "ding" you get a message.
Settings>Messages>Receive At>Caller ID
Is where when you send an I message to an iDevice and they dont yet have you programmed into their phone as a contact THIS WILL BE YOUR CALLER ID. so change it accordingly. iPhone users will probably want to make this their phone number. Other iDevice users will have a choice between different emails not a number if it's a pod or pad or Mac.
If you send an iMessage to a non iPhone iDevice your email will be used as caller ID instead (because they dont have a wireless number associated)
IF YOU LOSE DATA SERVICE EVERY NOW AND THEN, THEN THIS PART WILL MAKE LOTS OF SENSE
If you originally start an iMessage using an iPhone users email and not phone number, then for whatever reason if you don't have data at the time of your message it will send it as an email NOT A TEXT. It does not revert back to their default number even if it's stored into your contacts.
So if you are in the mountains or under a bridge and you have crap for service aka no data only cellular and you send an iMessage it will go to that persons email address that you originally used to start the conversation from something like email@example.com NOT THEIR WIRELESS NUMBER AND WILL NOT SHOW UP IN IMESSAGE.
It's dumb but I figured it out this while AppleCare was scratching their head last night.
Settings>Messages>Send As SMS
This is the exact opposite of the conundrum from above and is more straight forward. If you don't have data when you send a message to an iMessage users phone number then it will send as a text message and cost u money if it isn't unlimited or whatever. So unclick if you are a text fiend with a low text plan.
hope that covers 99% of the questions about iMessage.