This might more properly belong in a Pirate Islands thread, but I think AMD will have interposers in 2014. It was supposed to happen in late 2012, but evidently it didn't.
I don't know why Tiran was scrapped. Maybe the technology just wasn't mature enough to guarantee decent yields, maybe it was too expensive, maybe AMD's progress with memory PHYs made it a bit less attractive.
But if you look at Hawaii, it's pretty clear to me that it needs to happen for the next big GPU. I mean, suppose its 20nm replacement has 64 CUs (4096 shaders), where is it going to get its bandwidth? AMD is already running a 512-bit bus with 5 GT/s RAM. Presumably they could push that to 6, maybe even 6.5 GT/s, but that would only be 30% more bandwidth at a very significant power cost. The alternative would be a wider bus, but 768 bits? That would be pretty damn costly, not to mention very large on the die.
So it seems to me that interposers need to happen concurrently with the jump to 20nm.