I just upgraded from Confluence 3.1/Builder 3.3.5 to Confluence 3.3/Builder 4.1.3 and now my text is being centered. I verified that Page Alignment is set to Left in the Options tab.
It appears that the atb-content.layout css is defining:
And that is impacting my page formatting. When I change it to align:left in Firebug, it resolves the issue. Is there any way to change this for the entire layout?