imessage

Hi.

imessage works fine between my iphone and my girlfriends iphone. But, I was under the impression that I should be able to continue a conversation on eg my ipad. However, there seems to be no syncing between units whatsoever. I can send a message from my ipad and my girlfrined will recieve it, but it wont come up on my iphone. Same, if I send a message with my iphone it won´t come up on iphone.

What am I doing wron ? (all units are using ios5)

Thanks in advance,

Nigel