Hello Jeff,

I've found the following paragraph in C3750 config guide:

The default configuration for local SPAN session ports is to send all packets untagged. SPAN also does not normally monitor bridge protocol data unit (BPDU) packets and Layer 2 protocols, such as Cisco Discovery Protocol (CDP), VLAN Trunk Protocol (VTP), Dynamic Trunking Protocol (DTP), Spanning Tree Protocol (STP), and Port Aggregation Protocol (PAgP). However, when you enter the encapsulation replicate keywords when configuring a destination port, these changes occur:

•Packets are sent on the destination port with the same encapsulation-untagged, Inter-Switch Link (ISL), or IEEE 802.1Q-that they had on the source port.

•Packets of all types, including BPDU and Layer 2 protocol packets, are monitored.

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_44_se/configuration/guide/swspan.html#wp1200733

So you need to add this option to the destination port.

Be aware that if you are using Remote Span this is not enough to see BPDUs

Hope to help

Giuseppe