v6 & DSL / Cable modems [was: Private use of non-RFC1918 IP space (IPv6-MW)]

All IPv6 address assignments are leases. Whether you get
  the address from a RIR, LIR or ISP. The lease may not be
  renewed when it next falls due. You may get assigned a
  different set of addresses at that point. You should plan

  The only difference is the mechanisms used to assign the
  leases and the probability that you may have to renumber
  when the lease falls due.