How about a recognisable logo such as a picture of a finger putting a coin into a slot?
I don't think "warning" is the word exactly, but yes, I'd certainly agree with this. Not just from the point of view of whining about the state of the games industry, but also the possibility of kids buying loads of stuff ingame without their parents realising. This should be mandatory.
I mentioned this in the Gran Turismo comments, but I think having a micro-transactions warning on the box if retail, or when you confirm the download if purchasing online should be mandatory. It has gone too far now.