If I'm reading this correctly (and I think I am), this means that 15 nights with this promotion earns Diamond through Feb 2011, but you still need the full 50 nights to earn Diamond through Feb 2012.
This being the case, and being that 180 days is about 6 months, registering for the promotion after August 2010 would still mean instant Platinum, but would not provide guests any incentive to reach the 15 nights for Diamond (or 5 nights for Plat) through Feb 2011 since the promotion provides that already.
Just thought this was interesting, as the promotion allows you to register through Dec 2010.