Synchronous Optical Network (SONET) is a high-speed TDM physical-layer transport technology, inherently optimized for voice. Packet-over-Sonet (PoS) provides a means to use the speed and excellent management capabilities of SONET in order to optimize data transport.
This can be required in a scenario, when you try to use RSPAN to monitor voice over IP traffic from the other locations. You cannot monitor the source VLANs and dump the traffic to the RSAN VLAN. The problem appears to be RSPAN over the sonet ring. Locally you can see the traffic, but not over the sonet ring and on the core 6500 switches.
The RSPAN works over Layer 2 Ethernet frames, therefore it does not work over SONET.
You cannot span a PoS interface as it is a Layer 3 interface. You can only span Layer 3 interface that can be used as a Layer 2 ports as well. In other words, you can configure both Layer 2 LAN ports, which are ports configured with the switchport command, and Layer 3 LAN ports, which are ports not configured with the switchport command, as SPAN sources or destinations.
Since you cannot configure Packet over Sonet ports as switchports, they cannot be configured as source or destination ports.