Here's some details on the new Death Knight class that is coming with the expansion...

The Death Knight will be a plate wearing DPS/Tank hybrid. Once you have hit level 80 with your main character, you will have the option of undertaking an epic quest to ‘unlock’ your Death Knight character. Supposedly any race can be a Death Knight and you will start out as a higher level character, probably in your 60 - 70. The Death Knight is considered to be an alternate, advanced class to be used in end game play only.