Winreview.ru’s latest Office 15 screenshots reveal application-specific border colours and drop-shadows. Word gets a dark blue border and shadow, Excel gets a green border and shadow, etc.:
Microsoft in fact applied for a patent for per-window glass colourisation in late 2005 (‘Glass appearance window frame colorization’; discovered by Long Zheng in 2007). While there isn’t any glass in the Office 15 windows shown here (despite the DWM being enabled), the idea is similar.
Office 15 and Windows 8 seem to be moving in similar UI directions – it seems like square corners are in and transparency is out (at least for the ‘Aero Lite’ theme).