Windows 8 has now decided that all captions should go in the centre. (Office 2007 used pale captions that looked like the window never took the focus. Seriously WTF.)
Caption position is just another of those inconsistent things - in Windows 8, caption is actually in the centre between the upper ribbon tab marker (which highlights the tabs specific to currently chosen object) and the window buttons, while in Office 2010 the marker is ignored, and the caption is instead centred in the titlebar. I actually find non left-justified titlebars somewhat annoying in Windows 8, because things move around, and I have to scan around to find the information that was previously in a fairly static place.