Since I was a bit curious about what was going on here, I just gave it a try.
Configured a SPAN session like so
sw01(config)#
monitor session 1 source interface Gi0/8
monitor session 1 destination interface Gi0/3
The result is, the line protocol of the destination port is advertised as down
sh int gi 0/3
GigabitEthernet0/3 is up, line protocol is down (monitoring)
...
Acording to (a bit older) pice of Cisco documentation this is to be expected
(on the destinarion port) All input traffic is discarded and a value of DOWN is advertised as its operational status to all applications running on it.
If I remove the destination for the monitoring session, the interface is comming up again
sw01(config)#no monitor session 1 destination interface Gi0/3 ingress dot1q vlan 1
sw01(config)#do sh int gi 0/3
GigabitEthernet0/3 is up, line protocol is up (connected)
Cheers
Thomas