I actually blame Nojima for that one, turning Nami into all but a second Kairi :\ end of Reverse Rebirth - it seems she's capable quite a bit to care for Riku as well, and I doubt it was "just" because he was Sora's best friend.
Well, to be honest, I haven't reached the end of Reverse/Rebirth yet. I'm still only on the GBA game, and the 12th floor. I can't get past Riku. The one time I got close, my batteries died. Stupid console. Grr.
Even if she did seem like she cared at the end of CoM, a year does take place between CoM and KH2. So, is it possible she could have changed more than her looks within that time?
Also, think about her promising Roxas to see each other again. That was a different promise than the one she gave Sora at the end of CoM (them making it to be one and the same is again Nojima's fault and Nomura's this time, but besides my point). Nami all but pulling Roxas out of Sora at the end of KH2 shows she did have some concern for anyone other than Sora, and in fact, if she wanted Sora to be whole whole whole forever, pulling Roxas out of him and them promising to continue being themselves through him and Kairi would be rather counter-productive.
She'd go far, but to a limit.
What Naminé shows to Roxas is completely contradictory. She tells him that he's not supposed to exist, apologizes, basically shows him that he's heartless, and then promises that they'll see each other, a promise that she shouldn't have known she would be able to keep. She made the promise because she knew that she was Kairi's Nobody, and that Roxas was Sora's Nobody, and that they would merge with their corresponding Somebodies.
And really, does it say anywhere that Naminé actually pulled Roxas out of Sora in the end? Roxas was already 'awake' within Sora, so wouldn't he have been able to pull himself out, once he saw her? Also, this completely contradicts her early behavior of 'Sora, Sora, Sora'.
So, perhaps it's Nojima's fault (although I still stand that Watanabe worked in some capacity on KH2, and since we don't know what he worked on, we can't really say that it's all Nojima's fault), but either way, the damage has been done.