I'm playing 1.9 prerelease 4, and I'm having trouble getting the looting enchantment. To test this to see if it was just bad luck, I made a separate world, INVedited in 64 diamond swords, tons of book cases, an enchanting table, and used NBTedit to give myself 99999 levels.
I placed the enchantment table, surrounded it with book shelves, and started enchanting all of my diamond swords, always picking the highest amount of levels required to enchant. I have gotten multiples of every single enchantment in existance, except looting. No matter how many times I try, I still have not gotten my hands on a looting sword.
Hover your mouse over the sword after it has been enchanted, and it will tell you what enchantments have been put on it. Unfortunately, there is no way to know what enchantment you're putting on it until it's too late.