The tip in the web discussion section on the forum that mentions how to center a page in QID but it works in the web layout... the very simple and short version is this...
1) On the new page dialog or the layout properties dialog, make sure the "Variable Width Page" checkbox is checked.
2) On your master page, insert a 1 pixle wide text box on the left and right side of your pages that run the entire length of the layout (you may need to adjust the length of these boxes on each page created by that master page because the page length will change depending on what content you insert on that page...).
3) Go into the modify dialog for each text box, under the text tab choose the "Make Variable Width" checkbox.
4) Layout your pages using that masterpage or update your existing web master pages with these changes...
Now, as was noted in the previous posts regarding this issue, you must:
make sure those boxes are as long as your content (which may need to be modified per page depending on what content is on that page)
make sure your content at the edges of your pages does not touch or overlap those boxes
Think of those two boxes as bumpers which grow or shrink to push your content to the center of the box. It's not the most elegant workaround but it should work and we are working on actually fixing this in future versions.
Hope that helps!