There were rumors about that.
I don't think the "point" of the Macbook is having a cheaper system, which in Apple's case unfortunately also fills in for their tiny notebook.
There was a 14" iBook, so I don't think it's out of the realm of possibility to see a 15.4" Macbook. It's nuts that Apple makes you spend $2000 just to get a 15" screen, when other PC makers have 17" models for well under $1000.