iMessage question

Just upgraded my iPhone 4 to IOS5 and I have a question about iMessage which I have not seen covered here.

When sending a message to my gf, she is receiving it as an email message instead of receiving it in the messaging window.

She is using an iPhone 4S and this issue only appears to be happening with me. Messages from other IOS5 devices are showing up in blue in her messaging window. However, mine go directly to her email inbox.

Any suggestions?