Hi
Could you clarify a few things.
1) The FWSM is between the ISP and the 4500 ? If so is it running in transparent mode or routed mode and if routed mode is it participating in multicast routing or not ?
2) The multicast stream from your ISP. Have you checked the ttl setting on the multicast stream ?
3) The etherchannel between the 6500 and the 4500 - L2 or L3 ?
I'm not aware of any inherent problems with multicast and etherchannel as the etherchannel link is just seen as one logical link.
Jon