Usually, when there's a sale like this on a normally fixed and highly priced item, such as the HD800s, it means a newly upgraded model is soon to appear. And, with the somewhat recently introduced HD700s, it increases the likelihood, i.e., HD900. Plus the HD800's have been around for quite sometime and I'm sure Sennheiser is more than ready for some added income to their treasury.
Well, I've read rumors that Sennheiser is working on next coming model, but they
won't release it too soon since the hd800 still sells well.