I had this same issue before...working with CS I tried to buy a ringtone three times, each time I coudn't open the message; also went in to the local store.  CS took off the charges easily enough but basically indicated they didn't know why the text message wouldn't open; kept citing technical issues (yeah, I had figured that one out myself).  I  gave up trying to get a different ringtone but I miss the one I had with my original DROID, which was a third party tone.