Somehow someone managed to get his hands on some Halo 3 ending video. He said and I quote
I managed to scam these out of the guy that was selling h3 on ebay early.... All I can say is... wow, What a way to end a trilogy, sent shivers down my spine
Whether this is true or not, no one can confirm it at the moment. But if you see the YouTube link below go poof, I guess you got your answer. So watch it while it lasts. Even though it may be fake.
Edit: By the way, the poster (Enraged Toaster) setup a rapidshare link for you to download the Halo3 ending video in 3gp format.
Edit2: He has another link in wmv format.
And if any of the links go down, you could try tk Video Search (based on Google Custom Search).
But here's an alternate link: Halo 3 ending at Youtube (may contain spoilers)
Edit: You may also want to check out Halo 3 wallpapers here.
[Source: Halo3 Forum (may not be work safe)]