My concern is that obviously we can not use all the native 800x480 pixels of a 3-3.2" LCD widescreen.
For example on a 3" widescreen (3.2" actual) LCD with 800x480 resolution the viewable area measures 67-68mm wide (depending on manufacturer and model). Now if we measure the inner width of the "Finder" mounting frame - it is 62mm. So if the width of view of a mounted finder is 62mm and even a 3.2" screen is 67mm we lose 5mm total on sides that translates to 740 pixels of usable horizontal resolution even on this 3.2"ish screen.
It's a tricky question as 5D MKII and 550D/T2i output during recording is quite far from using the actual width of the screen and when re-scaled to that you lose some on-screen readings, nothing cruicial but still...
BTW the Z-Finder without mounting frame measures 67mm inside and can be used on 3.2" screen but then you lose the elegant way of mounting it.
Update: 3" sample screens we got measure almost 3.2" and these have visible area of 67mm X 42mm.