Dpreview has an interview with Robert Cristina (Manager of professional products, Europe) and Ludovic Drean (Product Manager for lenses, Europe). Drean said:
"If even 5% of the D40, D50, D60, D80, D90 owners buy this lens, that's a huge number," - this pretty much explains why this lens was released - it has a huge financial potential for Nikon. He also said: "this is not the last lens announcement we'll be making this year."
I know, this is obvious but somehow I got the feeling he was referring to prime DX lenses.