How to iMessage internationally?

Hello everyone,

I live in the United States and my best friend moved to Japan. She recently got an iPhone with a cell phone provider over there and has an iPhone 4S with 3G capabilities as do I. I was under the impression that iMessage was similar to BBM for Blackberries where if you know the pin (in this case the cell phone number) of the other person you can send them free messages regardless of where they are. I have her Japan number and I have entered it using the country exit code and Japans code and have not been sucessful in sending the message. The send button still remains green and not blue. I have tried various combinations of saving her number in my phone and none results in the icon turning blue. Is it possible to iMessage internationally? I am a former blackberry user and I am so glad to have an iPhone because it is soo much better, and I imagine that iMessage is designed as an alternative to BBM for iphone to iphone devices and hope to be able to send texts to my friend over in Japan via iMessage. If anyone knows how to set this up, it would be awesome!

Thanks!