I don't know how true this is, but...

[ReaperFett]

From IMDB:

Liam Neeson was to make an appearance in this film as a spirit just as Alec Guinness had done in Episode V and VI, but injuries sustained in motorcycle accident in early 2000 prevented him from working. Instead his voice can be heard in Yoda's thoughts right after Anakin kills the tribe of Tuskin Raiders.

[JMK]

I can see it as being true, but I won't call it a fact yet.

[JediBoricua]

I read some reports about that accident, and it was a very nasty crash. Maybe that's true.

If true, we might be seeing him in the Ultimate Edition DVD's, meesa thinks.

[ReaperFett]

Well, I remember Liam hinting he was in EP2 ("Lets just say my schedule is free in ____" or similar)

[Jedi Master Carr]

It might be more speculation than anything else, but it could be true.

[Nirvana]

I remember he hinted he would be in EpIII, but not EpII.

[JMK]

I've always believed that we had not seen the last of Qui Gon. We heard him in AotC, so a natural progression would be to see him next. Remember, we heard Kenobi twice in ANH before we saw him appear in Empire. Lucas loves to make little parallels like that. :)

[Hart]

Goooooo Lucas!