I'm seeing the same behaviour described in Preview eats first and last $ so ignores displaystyle, an old bug report that's been marked status-completed for over a year. This started about a month or two ago, and it only occurs in Safari (version 5.1.1 (6534.51.22)), not in Firefox (version 8.0.1).
In the preview, equations surrounded by double dollar signs are displayed as if they were in single dollar signs, i.e. left-justified and without display style. (If I add a third dollar sign in front, they get displayed properly, but preceded by the extra dollar sign.) This occurs only in the preview.