Ah... I hadn't thought of areas that split up into very distinct sites :)
That's a good point.
But looking at
http://live.gnome.org/GnomeWeb/Navigation, the only areas that split into distinct sites are 'News', which really only splits into pages, and Development, where each subsite will have a distinctive header.
In short -- I don't wholly disagree, but I'm not sure it's worth tinkering with the common bar for this -- either in terms of looks or technology.
(Didn't we say we'd transclude the HTML into sites so we guarantee updates to the bar design roll through all sites magically?)