This is the normal behavior of the destination port in SPAN session that the usual traffic will be blocked on the dest port while you get the data from the source port. If you put a sniffer on this port (like Ethereal) you can see all the traffic which is sent or received by the source port.
When span is active, incoming traffic is disabled. The port does not transmit any traffic except that required for the SPAN session.
It does not participate in spanning tree while the SPAN session is active.
When it is a destination port, it does not participate in any of the Layer 2 protocols (STP, VTP, CDP, DTP, PagP).
No address learning occurs on the destination port.
one option is to have two NIC cards in your system so that one can be configured as the destination SPAN port and the other can be used for establishing connectivity to it.
HTH
Narayan