This has been discussed before, and I believe the official word is that releasing TNG in high def would actually be a much larger undertaking than TOS. The reason is that the special effects for TNG were shot on video, not film.
I think it was the effects were shot on film, but edited on video and the original film foorage was destroyed at that point. Works out the same, though.
I could have sworn that I read somewhere that this isn't true, that the original film stock is still available, but I can't find that information at the moment. Certainly at least some
of it is still available, as some original TNG 35mm footage was used in the Enterprise finale and it was definitely not sourced from video tape. This was covered in the original thread here
I found this link
which is the convention information I was thinking of. John Van Citters (who was speaking) confirms that they are actively pursuing ways to convert TNG to high definition but does not mention whether or not the film stock is available. He does however say that it would be "incredibly difficult" but that he "expects it will happen eventually".