Have one that isn't zero.
Please. I don't want to resize my browser (currently 925px, which is reasonable) to make your content readable.
This is what I'm talking about: http://i.imgur.com/Dlcgg.png
Once, long long ago, servers on the world wide web simply sent me data (text, links, and maybe images) and I could format how I pleased client side. I think we have gotten too far away from that original model. (These days I have to run your client side code to download to insert text in the DOM. On top of some ugly background. With margins unusable on my devices. If you want to serve me some text, just send the damn text).
Yes, it is nice that a web designer can suggest to my client how that text might best be rendered, but when it comes down to it I actually know how I want to read a document; hint: it probably doesn't match your layout.
[ / end rant ]
* Yes, I understand _actual_ applications exist on the web now; rant applies only to "articles"
940 is a commonly used width content for many grid based frameworks. That 940 doesn't include an internal margin, instead it is handle by an external 10px margin taking the whole width upto 960px.
Check out 960.gs for their framework, but they aren't alone in those figures, because of the way it subdivides.
Actually I would rather use a browser width of 600-700px, as that would fill around half the width of my 13" laptop screen, and be more typographically optimal in terms of not requiring as much horizontal eye scanning, and not stretching out paragraphs into 2 or 3 line horizontal stripes. But there are too many websites out there that assume I have a giant monitor and a browser window that's maximized to fill it.
As for the iPad... it scales websites to fit its format, so I'm not sure what the issue if there.