Here are some of the rumours with some additional-extended details (more then it is available on the wikipedia page) - I would really like to see this issue resolved in the future. Such a shame if the original negative really exist and it is still unreleased :(
Source: http://encyclopedia.wizards.pro/index.php/Return_of_the_Ewok
Evidence of the original print
The original 16mm print was said to be lost according to Davis, the only copy in existence (though bootlegs now exist) is the one Davis holds in his possession, the supposedly one and only VHS tape copy of the movie. A copy of the print, or even the original print, may actually reside in the Lucasfilm archives. Evidence of this was shown on the 2004 DVD release of The Star Wars Original Trilogy. The segment of Wicket being chased by Boba Fett was located as a segment in an Easter Egg of bloopers from the trilogy. It appeared in a widescreen format in good quality, suggesting that it was filmed this way and has been preserved. Another piece of evidence that would lead one to believe that it resides there is that a small segment was used in the 2004 DVD in the first place. It being such a small and insignificant, and hard to find part of the DVD, it is highly unlikely that Lucasfilm could have used Warwick Davis's copy, or even got a copy of it from him on such short notice. And even then, the clip was so short, it would seem to be a rather small, quick idea that the producers of the DVD came up with in the DVD set's post-production.
It is also possible that the original print lies in the archives due to the fact that the archives have been reported to contain nearly every single piece of star wars artifacts ever created. And it is likely the Lucasfilm crew kept it, in order to use on future projects, as they have been known to with other media. A final possible, though not necessarily true piece of evidence leading one to believe the original print exists is the 2005 starwars.com release of four minutes of footage from the film. Once again, it is unlikely the footage could have been easily and quickly obtained from Davis when the clip was released.
Celebration III Evidence
Warwick Davis said at C3 that he loaned the VHS tape of his to Lucasfilm, who made a digital copy of it, and that it was the best copy that is known at this point as well as being the source for the Hyperspace clip. Based on the quality of this copy seen at starwars.com, they did a minimal amount of cleanup on that copy. The editors also altered the soundtrack--where there used to be Supertramp's "Take The Long Way Home" as temporary soundtrack, they've dropped in other incidental music (to avoid copyright clearance issues). Also, they seem to have tweaked the soundtrack in other places, and in at least one instance a line is missing (Wicket saying "Crikey, it's Lord Vader!" right near the end).
2004 DVD Clip
There have been several fan-based rumors as to how Lucasfilm obtained or created the clip from Return of the Ewok that was found on the 2004 DVD release of the Star Wars classic trilogy. Good question. One theory is that either Lucasfilm took Warwick's VHS copy of the footage and digitally cleaned and restored it, and used that. (Though this is not likely, as the quality appears way too good, even for Lucasfilm's expertise). The other theory among fans is that there was a reel of film in their archives with that particular scene on it (much more likely). There's also the little piece that appeared in the VH1 special (Wicket saying goodbye to the heroes), which I figure is likely the same story. Another explanation is that Return of the Ewok was shot at the same time as Jedi obviously, so footage intended for it appears on the "making of Return of the Jedi" film reels, and even on the actual film reels for the film, in the case of the Boba Fett clip. It would be simple enough to construct a very nice looking 35mm film print of the film, if Lucas ever wanted to, and assuming most or all of the footage has survived.