I'm amazed I can't figure this out - my friend has an iPad running iOS5. He can send me an iMessage to my phone using my AppleID. But (outside of replying to his message) there is no way for me to send him a message using the Message app, since it only lets me enter phone numbers.
So, how does an iPhone user send an iMessage to an iPad user's AppleID?
EDIT: Does this work for anyone else? ON AN IPHONE (3GS, if it matters, running iOS 5), open Messages. Hit the 'Compose' button. Try to type in an email address in the 'To' field.
There is NO WAY to enter an email address there, afaict