I know of plans for at least 2 exchanges that will use 100baseTx
as a connection option. The only real problem is
the backplane limit of boxes that switch 100baseTX,
(Cisco Cat 5000, 1.2gbps). This is only slightly
better than the Dec Gigaswitch limit of 800mbps.
Certainly the hardware to do switched ethernet is much cheaper
than FDDI, and with full duplex 100mbps there is very little
advantage to FDDI. (FDDI does do full duplex now, but I don't
know who offers interfaces that support this.) FDDI
does offer larger frame sizes, assuming you have FDDI each
end with no "MTU 1500"'s in between.
To me it seems like a big cost win to drop in a C5000
with 4 slots at 24 switched ethernet/slot, or 12 switched fast
ethernet/slot. Particularlly since the router interfaces are
about 1/2 to 1/4 as much.
The exchanges with the most traffic are the ones
based on the oldest technology, because there are always bugs.