If you want a monitor that will display true HD, it must support a native resolution of 1920x1080 or higher , checking the 20inch monitor section, i think only the Dells support this.
If you want to be able to view HiDef content from HDDVD and Blueray (possibly more in the future) then you will also need the display to support HDCP for any content which is protected by the ICT (Image Constraint Token).
As already mentioned i think most movie studios have agreed not to implement that until around 2010. However if there protection is partially broken quickly, then expect to see the ICT rushed into service to try and combat it.