It is in fact, Diablo 3 supports all resolutions and formats.

Bashiok confirmed custom resolutions:

We're supporting all the normal and widescreen (16:9 / 16:10) resolutions. Switching from a normal aspect to widescreen you will see more playable space, but within each aspect ratio you're not going to get more game space info. That's how I understand it works right now anyway.

So, personally, I think a widescreen monitor would be a good investment. For a lot of reasons/games really. But for Diablo III a 30" 16:10 isn't going to let you see more of the environment versus, say, a 19" 16:10. You'd just have the visual quality improvements that generally come with a higher resolution.