It's an interesting one. For a while I wondered if DVD / BD would pan out like CD / DVD-A SACD.
I thought now people had bought into having the slim screens and the players and disks were coming down in price, then BD would take over and DVD slowly disappear.
However, as it stands, all the big films still seem to appear on both at the same time, there's a much bigger DVD back catalogue, and the DVDs are significantly cheaper. Though I haven't gone hi-def yet, when I buy a DVD I do wonder if I'd be prepared to buy a BD if I could get 2 or 3 DVDs for the same price.
The thing is though I know I'd get better picture and sound, my standard definition setup seems very good, and I'm not sure how much more I'd enjoy films on BD, especially considering the fewer I'd see. I've recently watched Breakfast at Tiffanys and Sherlock Holmes at the pictures and made a special point of looking at the picture and assessing if it showed me more detail than on DVDs at home, and I really didn't think it did, or that it would enhance the film much if it had done.
I think if DVDs continue to be produced then it may well pan out to be like CDs. I think they might need to phase DVDs out to an extent, as happened with records, if they really want the shift to come. I don't think they help themselves with talk of further higher def formats and 3D either which has also helped put me off upgrading for the time being.