On a related note: the SEND CGA stuff mandates using the subnet prefix in creating the interface identifier and as such makes it impossible to have the same interface identifier in different subnets. I was unable to convince them of the error of their ways and apparently there was no IETF last call or I missed it so now this stupidity is an RFC. We should do our best to make sure there isn't any more of this.
I happen to agree with you and disagree with Christian - there should
be a mode in SEND in which the CGA address is generated without including
the (typically /48) prefix which will vary between ISPs when multihomed.
For multi6, I believe we should not feel constrained by this. It would be a fairly simple extension to SEND to allow this, and it might be a very valuable functionality vs security tradeoff for a multihomed site that wanted to run SEND.