Mostly, web design was aligned on a 640 x 480 pixel screen with the usual borders for location bar, navigation buttons and such. That's the reason for the standard 468 x 60 ad banner.
Then came 800 x 600, followed shortly after with 1024 x 768 pixels. 1024 is still popular as the majority of LCD projectors has this physical resolution.
On the desktop, wide screen formats and resolutions of 1280 pixels in the x axis dominate a lot of web designers aim for the total desktop-width domination with screen layouts that are unreadable on smaller screens.
But with a lot of mobile devices, ultra mobile pcs, web tablets, iPhones and such, the tiny screens return. And I suppose that the users behind these cutting edge, early adopted, geeky devices are a most interesting target group for advertisers and content producers.
So, does your web design work on 800 or less pixels width?