So, an update to my previous image to avoid any confusion about what the images are showing:
The mystery port is some kind of video out - not USB-B, not a combination of USB and Firewire, not Mini-DVI, not HDMI or Mini-HDMI, not (full-sized) Displayport ... but it may be connected to
this PDF discussed earlier in this thread that includes three mentions of a "mini-displayport" being developed specifically for Apple by the Taiwanese 'Longwell Company' between January and March of this year.
See page 43 (or just search for "mini displayport")
(c) Signal cable assembly sets:
Year 1/1/2008-3/31/2008
R&D Accomplishments:
3.Development to Apples Mini Displayport.
In case the inline PDF link is broken, here is the full link:
http://www.longwell.com/download/Annual Report 2005.pdf