IF they are doing SONET, DSX, or some other type of enterprise circuit then yes. We have several racks like that from carriers in our datacenter.
"You don't want anyting between your router interface and the bgp neighbor's interface since you want to be able to catch linkdown events and not have to rely on timeouts."
Then you don't need to use DS3 or OCx interfaces. The only thing you will be able to detect is a local loss of connectivity. If the far end of the link looses connectivity your router will not have the first clue the far end is down. If you are using a DS3/OCx interface if either side goes down you will detect it. If you are using ethernet you will only know if your side is down.
Not all customers. Some may just get an ethernet switch, others a simple media converter, others plain fibre handoff. Its highly dependent on existing infrastructure, or the carriers particular standards for provisioning certain types of services or circuits.
e.g. an ISP I used to work for back in Australia had some ethernet services brought in from a carrier to aggregate ADSL customers and provide Internet connectivity.
For the ADSL aggregation the carrier installed a Cisco 3550-12G as the CPE for that service, while the Internet service was a plain fibre hand off.
Both were installed at the same time so it wasnt just a change in standards at some point in time, it was just how they did different circuits for different purposes.