Feature films, Blu only. These are generally titles I will generally watch soon after purchase, keep (or sell if it was a blind buy gone terribly wrong), and rewatch. If I really want to see a movie not out on Blu, I will rent it.
TV series, I will still buy DVD when a "complete series" set of something that I may have only seen some of comes along at a ridiculous price, 25-50 cents per episode. These I may not even start until six months after I buy them, and I may watch them elsewhere than on my HT system. Recent examples: Six Feet Under, SG1, Ally McBeal.
The exception being series, like Firefly, BSG, Rome, Band of Brothers, and a few others that I expect to become staples. Even then, I will do without until the price comes down.