iMessage uses your phone's data connection. That data connection may be cellular data, or it may be wifi. It doesn't matter to the application how the data connection is provided, just that it has a data connection to the internet.
Now, a few facts about iMessage. When an iPhone user enables iMessage, and sends messages to non-iphone customers, or iPhone customers who have not enabled iMessage, those messages "fall back" to send as SMS, so they will appear on the monthly bill. If an iPhone user sends a message to another iPhoen user who has iMessage enabled, but there are connection errors, the message will fall-back to SMS (unless explicitly disabled in the settings). Those messages also appear on the itemized bill showing the phone number the message was sent to/received from. In all of these cases, the mesage bubble on the iPhone should show as green, indicating that the message was sent/received using SMS.
In the "normal" iMessage usage (iPhone to iPhone), the message bubble appears in blue, and the message uses the data connection (wifi, cellular, whatever). For these messages, the AT&T bill does not itemize the messages and does not list the remote phone number (because not all remote devices are phones so they may not have a phone number). This usage will be grouped into the summarized data usage (that is rolled-up into 3 hour increments for Mobile Share users). AT&T has no way of knowing who the remote message was sent to/from so they can't itemize it on your bill, nor can they offer any kind of filtering of that usage, other than disabling all cellular data, because to AT&T that's all it is.
Now, a few facts about iMessage. When an iPhone user enables iMessage, and sends messages to non-iphone customers, or iPhone customers who have not enabled iMessage, those messages "fall back" to send as SMS, so they will appear on the monthly bill. If an iPhone user sends a message to another iPhoen user who has iMessage enabled, but there are connection errors, the message will fall-back to SMS (unless explicitly disabled in the settings). Those messages also appear on the itemized bill showing the phone number the message was sent to/received from. In all of these cases, the mesage bubble on the iPhone should show as green, indicating that the message was sent/received using SMS.
In the "normal" iMessage usage (iPhone to iPhone), the message bubble appears in blue, and the message uses the data connection (wifi, cellular, whatever). For these messages, the AT&T bill does not itemize the messages and does not list the remote phone number (because not all remote devices are phones so they may not have a phone number). This usage will be grouped into the summarized data usage (that is rolled-up into 3 hour increments for Mobile Share users). AT&T has no way of knowing who the remote message was sent to/from so they can't itemize it on your bill, nor can they offer any kind of filtering of that usage, other than disabling all cellular data, because to AT&T that's all it is.