I don't think that PM has ever been indexable? Though there used to be that static mirror that went away mid-way through last year that was indexable.
Can't but help wonder if one of the alternative urls couldn't be made to simply present the nodes as 'static' entities. With none of the expensive bits. No user auth or checks; no user css or preferences; No edit facilities or print/with replies/xml/top index/etc.; no nodelets or CB; no hierarchal presentations. Each node just displays that nodes contents amd nothing but. And have the robots file redirect them to that.
Would that reduce the cost of presenting a node sifficiently that PM could allow indexing without huge costs?
I know that if such a simplified view of PM existed, I'd probably use it in preference to the current view for most things.