Dual-format players that cost only a little more than a single-format player are quite probable in the HD-DVD vs. BD war. With VHS and Beta, it was all or nothing - the technology didn't lend itself easily to a dual-format system. With the HD war, you're dealing with spinning, shiny, dimpled disks of plastic being read by a 405nm laser. Sure, the disks need different optics for reading, and the data's arranged a bit differently, but it shouldn't cost that much more for a dual-format player, once the tech is more mature. I think there are already two dual-format players available (or nearly available) on the market.
Wouldn't it be funny if this whole format war never materialized? All that effort by Sony to develop a competing format, and it goes nowhere.