On luke's phone running beta 6, pinch zooming on large pages (lots of vertical space) gets worse lower down, as if it isn't centered properly, for example on tbpl.mozilla.org. Lower parts of the page make this worse. I don't see it on nightly however.
Do you mean Fx7 Beta from the marketplace? Does this also happen on Aurora? I don't think we would attempt to fix this on Beta. Even Aurora would be pushing it.
See more details and video in Bug 681662