Prompt when sending text message as SMS instead of iMessage? Is there a way I can get Messages to prompt when it wants to send a message as SMS rather than an iMessage ?

I hate noticing that when a message fails to send, its sent as SMS instead of imessage, which I get charged for. This happens when I activate the SEND AS SMS option.

However I would still want to send a message as SMS if necessary (emergency etc). Is there an option/app/tweak that can help me accomplish that?
This is something I wish iOS5 had. Alas, it does not - however if it fails to send as iMessage with send as SMS disabled, it will allow you to tap / hold (I forget which) and specify to send as text. 

The main caveat to this, though, is that it doesn't 'alert' you per-se.