Right, but why would Shaw put out IGMP onto a wire consisting of
nothing but "clients" -- home users? I can see them running IGMP on
the other (upstream) side of their router, but why talk IGMP to clients
when none should be talking IGMP?
Hosts speak IGMP, too. It's used to indicate interest in a multicast group. Normally the host would send something saying "hey sign me up for the stream at 229.1.1.1" and they'd start getting the stream. Every minute you'd then see a query to 229.1.1.1 from the router saying "hey local segment, is there anyone here that still wants this?" and it's the host's job to say "I do!". The 224.0.0.1 is a special case, basically a "hey are they any multicast listeners out here?" kind of thing.
Back to Occam's razor... It's probably a misconfiguration (if memory serves, it's just one command like "ip pim enable") or a field trial (IP TV?) and the address is again a misconfiguration or them using the address space for management.
Sean