The latest Mitsubishi Integrated HDTV Television sets have the HD Decoder included inside. While this allows you to connect an outside antenna and begin receiving "Over the Air" broadcast signals, it will not help you if you want HD programming from Direct TV or your local Cable provider. These "broadcasters" require their own decoder box, so you would not be using the HD Decoder inside the TV (that you paid for).
I recommend that you save some money on the TV and buy an HD-Upgradable Mitsubishi. You will still need to buy a separate HD receiver to receive the "Over the Air" broadcast signals, but it will make you more flexable when you are ready to upgrade your system.