[Web-storage] subdomains / cooperation and limits

I have searched the archives and been unable to resolve this to a great
answer and I just want to make sure that my understanding is correct lest I
have to unwind things later as someone has recently made me second guess
what I thought was a logical understanding of things.  Essentially,
x.wordpress.com and y.wordpress.com both allocate and use space - no
problem, right?  Access is subject to the browsers -general- sop, (leaving
aside the ability to document.domain up one), right?  If I have two
affliate sites who communicate across an explicit trust via postMessage -
is this problematic?  I thought not, and it doesn't seem to be - further -
I cannot imagine how it could work otherwise and still be useful for a host
of common cases (like the wordpress one I mentioned above).  I have been
told that the draft contradicts my understanding, but I don't think so.
Thought that some manufactures/maybe Hixie could set me straight...?

Brian