There is no special ending for finishing on Legendary, all you see is "Coming February 9th". Which many people speculate is when the rest of the game is released, either on Live or as an expansion pack.
"For the record: word from Bungie is that the 'Coming February 9th' rumor is exactly that - a rumor. A baseless, false, totally untrue rumor. I won't pass along the exact wording... but it was pretty clear in its meaning. Someone's pulling your leg. And by 'your', I mean the internet - even Slashdot was promoting this…