Actually, the female line of this clothing says: "Mary is my Homegirl". That's the one I saw Madonna wearing. Either way, one has to consider motive in this case. Clearly, Madonna and several other Hollywood types are doing this for other reasons than salvation...at least, that's the impression I get.
Now, that's not saying that T-Shirts are not a witnessing tool, or those who wear them are wrong. It depends on the message, it depends on the individual's motive, and their degree of committment to Christ. I have a plain grey t-shirt that says "I am the wretch the song refers to", and it's actually helped start several different conversations with complete strangers about Christianity and faith in God.
If it can be used for good, then so be it. If it's being used to be "hip", or some sort of shock-value mockery, then it's just wrong.