While firewire isn't dead the builting isight, shows up in the system profiler as a usb device.
I don't know if they'll add it to the display, but I think that apple will come out with a isight solution for the minis and the mac pros, and that the solution will
be USB.
There are actually a number of reasons to avoid the integrated iSight, one of which is that the lack of standard mics, cameras, or radio equipment, allows mac pros into secure places where those things are forbidden. And while it is true that they have forsaken that capability on their laptop line, there probably weren't enough secure area laptop sales to make it worth while.
Also the ability to use the screen as something other then a landscape mode Desktop monitor might make them less then eager to deface their screens with them.