I said no for one reason.
More selection is great for the consumer, as they can get something they prefer. If they don't like white or black, then aluminium would be a fitting alternative.
From a development standpoint though, more selection means you have to thin out your available hardware between the different styles, thus giving less hardware to the other available styles. And if rumors / reports are correct, the retina display in the iPad 3 is having low yeilds. With the current Black & White styles, your hardware is divided to the two styles, if you add aluminium (silver) to the mix, you're just making the number of each style go down for launch. Apple is only going to be able to have so many iPad 3's ready for launch in March. So throwing in a third style is only going to make it harder for people to get the one they want, especially if Apple anticipates a rush to the new 'silver' style, and it's not as big a hit as anticipated. Black & White models will be sold out all over the place, leaving an excess of silver models. Some people will buy them just because they don't want to wait for new stock to come in, but they won't be 100% happy because it's not 'the one they wanted'.
A silver iPad 3 might be something apple releases down the road once the initial demand for the new device has died down. Kinda the way the white iPhone 4 did. But I don't think it'll be available on launch day. And there's also the argument of skins. Don't like the way it looks, get it skinned.