Carrier Ethernet through the provider network is typically carried over SONET, a fiber technology, while between metro areas, the long haul, Carrier Ethernet is put into MPLS. This is all transparent to the enterprise since what you receive at your premises is an RJ-45 Ethernet connection. Supporting fiber to the building is preferred since it is a native transport, and fiber plants have been growing, but there are large swaths of locations still using TDM services over copper. Carrier equipment vendors are making gear to support Carrier Ethernet over traditional copper, including coax like broadband, and TDM circuits such as T1 and T3. Multiple TDM circuits can be bonded together to provide higher increments.
Getting Carrier Ethernet to more locations over copper is a benefit, but that leads us into geographic coverage. No carrier has a large footprint in the United States, at least. Providers such as AT&T, Qwest, and Verizon make announcements about increasing coverage into smaller population centers, but that leaves out huge chunks of enterprises that could benefit from Carrier Ethernet. The ability to interconnect with a local carrier will make services more broadly available. It's no surprise, however, that interoperation between service providers is still hit or miss.
Any service provider wants to own the circuit from end-to-end, for good reason. End-to-end ownership means that the service provider can manage the circuit more effectively and isn't reliant on a third party for support. It doesn't matter where a WAN service fails. If you buy it from your provider, you point your finger at them and you don't want to hear excuses about their provider issues.
Work within the Metro Ethernet Forum (MEF) is making headway to ease E-NNI integration and operational management end to end, regardless of who owns the parts. The excitement over E-NNI depends on who you talk to. Smaller service providers want E-NNI because they don't have the coverage of the big providers. Big providers are less excited because they're more likely to own the circuit. The standards work going on in the MEF should push interoperation forward. Let's hope the providers follow the lead.