WWE has been testing the implementation of HD for quite some time now, since September of 2006, at a live event in Uncasville, CT where they taped the show specifically in HD as a test show, complete with backstage promos and other things.

Vince McMahon, when asked at an annual stockholders meeting, predicted they would switch over to HD broadcasting within the year. He also commented on the quality of the product saying it "looks amazing under that format".

WWE has not announced when they will switch over, but SmackDown! will move to HD format by January 2008 according to announcements made by WWE and the CW Network. All TV broadcasts will be switched over to a digital format by 2009.

Source: PWInsider.com