Seems to me you're splitting hairs. If Cingular says, about three phones (not counting the Rokr), on their website that they're "MP3 players," and even go so far as to talk about the stereo headphones that comes with one of them, then they're marketing MP3 players. The fact that they don't have cardboard displays in the stores saying "MP3 Players This Way" doesn't mean that they're abiding by this purported agreement you've cited not to market any MP3-player phones other than the Rokr.
I still don't think such an agreement exists, and if it did, Cingular would already be in flagrant violation of it.
Heck, one of those phones (again, not the Rokr) is even made by Motorola.