iMessage not working between iPhone and iPod

Here's my current setup:

My wife and I each have an iPhone. My daughter has an iPod touch. All are running iOS 5. We all have our own Apple IDs and and have iMessage activated on each device.

iMessage works fine between the two phones, but I can't get it to work with the iPod. If I try to message the iPod, the send button is green instead of blue, and the message goes to the email account associated with the iPod's Apple ID. If I try to message one of the phones from the iPod, the name in the To: field turns red with an exclaimation point and the send button is greyed out.

I can't find any setting on any device that makes any difference. Has anyone had a similar problem and managed to figure out what's going on?