This has been discussed quite often already, see the search results on this for instance.
Shortly summarized the reason it still isn't available is because it is very difficult to incorporate it into the renderer due to its nature of design.
This doesn't clear things up, but only Matt could explain exactly why.
My guess for instance would be that the GI calculations are likely to differ, so one must find a way to store this cache.
IMO, there's not a desperate need for it at the moment, since TG2 renders pretty fast.
Unless you have a very complicated scene or use extreme render settings which are likely totally unnecessary.
Unfortunately the latter is the reason most of the times people have such long rendertimes.
In general you should be able to render a high def picture <10h in reasonable to good quality on a average spec'd machine.
Martin
