I'm no expert on any of this, but it's interesting that none of the sites that Chris has linked to provides any source for the Marie Roze story and, as we all know, disinformation spreads like wildfire on the Internet. The Wikipedia article on Carmen has 42 footnotes, but not one for this. The Wikipedia article on Marie Roze makes no mention of any approach by du Locle or Bizet (and it's interesting, en passant, to see there that she sang Leonora in La Favorita, like Carmen a mezzo role - maybe her Marguerite was transposed down?). The Marston site only says "Marie Roze is thought to have been Bizet's first choice. The short article about her in Grove doesn't say anything about Carmen.

One other thing: as Bizet died during the original run of Carmen, he can't have authorised any of the editions that came out afterwards. For example, recitatives (by Ernest Guiraud, who also monkeyed with The Tales of Hoffmann, replaced spoken dialogue for a long period and still do a lot of the time, and it's highly likely that sopranos wanted to sing the title-role and editions allowing this appeared.

Jon may have access to Winton Dean's book on Bizet - but if the Marie Roze story appears there, I'm surprised that Wikipedia didn't provide a reference. I think I'll slip a "citation required" tag into the Carmen article...