said by Sentinel: I am speaking not of a "networking" point of view but from a security perspective.
Perhaps I don't understand your question. If it is not being used then it doesn't matter. I don't know how else to explain it.
said by Sentinel: 1. How do I tell if my ISP uses IPv6?
Ask them.
said by Sentinel: 2. Whether they do or not, isn't it irrelevant if my router does not handle it, and thus all the PCs behind it don't need it either?
If you want to use IPv6 then your router must understand the protocol. Assuming your ISP supports IPv6 then the router needs to support IPv6 and so do the PCs on your LAN if you want to connect to IPv6 hosts on the Internet.
In a mixed IPv4/IPv6 environment there are different ways for the ISP to support IPv4 as they roll out IPv6. Remember the reason IPv6 has become so important is that we have used up all the IPv4 addresses. New user are not going to get Public IPv4 addresses, they are all gone, they are going to get private addresses from their ISP and the ISP will use NAT to map to routeable address. Similar to how home networks are set up today but on a much larger scale.
/tom