I know the technology industry moves pretty fast but the D3 released in July and now the D4 is coming out? It's barely been six months.
Six months in the technology world might as well be six years. Just because a new phone comes out does not mean that Motorola is going to stop supporting yours. It isn't even "End of Life" yet and you're worried?
Your D3 will most likely get ICS around the same time as the Razr and the Bionic. It shouldn't be hard for Motorola to get it working on the 3 phones that share a basic platform. D3 might even be easier since it's a 3G/world phone.