Centers ALL pages.
WARNING: Messes up the layout of many pages (including userstyles.org) that don't have a layout based on tables. To prevent that, remove the margin-left and margin-right settings (page will only be centered vertically afterwards).

Edit: Good to see this style is still liked (and still works in Firefox 3.0). And it doesn't seem to break this site any more, either.
Thanks for the reviews. I don't think you can add exceptions for single pages. Also, this style might break in future versions because changing the style of the HTML node is sort of a hack and is afaik not standards conform.
Well, whatever, keep enjoying it as long as possible.