Keep in mind that everyone has said the exact same things that we're saying, for the last 5 years.
However, it will level out somewhere, and I'm willing to lodge a guess that it will be around the current levels of travel. Motos levelled out at about 12" of travel and have been that way for years, and I believe that since we don't tend to need the same 12" of travel (less inertia and lower constants of momentum etc) that motos do, DH bikes will probably cut off in travel extension pretty quick. However, travel increases are a highly visible, easily achievable marketing ploy... some people don't realise that 8" of travel isn't necessarily better than 4" for a given situation (eg me, two years ago, wanting a 7" fork for my XC bike), so if you increase the travel on your bike, they automatically think it's a better bike.
Another thing: every time we drastically change travel on our bikes, we need new standards and capabilities from our bikes. For example, if you double the travel of your fork from 6" to 12", you have to compensate for overlap, sag, the extra leverage on the headtube, etc. If you increase the travel of your frame from 6" to 12", you have to adjust the geometry HUGELY, compensate for sag, run a shock that's twice the stroke (if not twice as long), beef up all the linkages that are now going to be twice the size, accomodate for larger changes in chainstay length (which is part of the reason the V10's chainstays are so ridiculously long, and which includes different hub spacings), make room for the longer shock, etc etc. It gets to the point where the weight of the bike is more of a defining feature than the suspension action. Look at Peaty, Nico etc... they're not running Monster T's for their plushness, they're running lightweight forks (Boxxer/BOS) for their weight. They aren't using 3.0 tyres for their pinchflat protection, they're using XC tubes in 2.5" tyres (and 2.5" is as big as the pros seem to go) to save weight.
It's at the point where the amateurs (read: most of us) are the ones calling for better suspension, not the pros. Peat's and Nico's bikes are both non-linkage singlepivots, which can only have a moderately rising/falling rate, and are generally fairly linear. Which just goes to show that the suspension action isn't as important as the ability to manhandle the bike.