I wonder how rare a Legendary item will be in D3
Depends how they've done the itemization. In D2, uniques were semi-common, but it was mainly the crappy uniques that weren't as good as the rares that you'd be using. The really good uniques were quite rare, and the really
really good uniques were extremely rare.
Then there was rune rares, before they upped the drop rate (and they are still rare then). I've always said that I have no chance to win a lottery because I used up my luck finding a legit Zod. Not that I ever used it for anything.