Doesn't seem to be so. I just upgraded. Lost my original connector & bought one off ebay. It did happen to be an Asus connector, the same manufacturer as my motherboard. I am running 8800 GTX 640Mb cards by two different manufacturers and that doesn't seem to cause a problem.
Still irritated that I can't get SLI to work while displaying Flight Simulator X across both monitors though.
most all connectors should work, the only issue is that some connectors have a certain length different from others. if you've got one of those flexible bridges you shouldnt have any worries about that.
the SLI connector for my EVGA 680i board IS NOT GENERIC. It is specific to this board because of the spacing the PCI-e connectors. In general, most board manufacturers space the PCI-e connectors the same distance apart, but not all do so don't take it as a general assumption.