While there have been strong rumors that Paramount was remastering Star Trek: The Next Generation in HD as they did with The Original Series, they have been just rumors; until today. LeVar Burton let it slip via his twitter feed that they are indeed remastering the series in HD:
Stopped by to see how the TNG conversion to HD for Blue Ray was coming along… #mindblown #bydhttmwfi
Where the Original Series remastering was pretty simple, the The Next Generation presents numerous challenges that would make the job more difficult and costly. Most of these have to do with the effects, such as the viewscreen on the bridge that often has people standing in front of it with effects in the background. The Original Series always had a more static screen so the effects were easier to replace.
The current rumor is that a four-episode sampler disc of TNG Remastered will be released by the end of the year with the first season due in 2012.