Life Tap is one of my favorite mechanics in the game. It's elegant, it's clever, and it's just plain fun; it's a big part of the reason a Warlock is my third-highest-level character (behind my two 70s). And it's really a defining part of the Warlock class, along with demons and soul shards. So when they nerfed it on the patch 2.4 PTR, I was mad, along with a lot of other people. I still am.
Anyway, two recent Eyonix posts have discussed the topic. First, he clarified that Improved Life Tap will work the same as in patch 2.3: you get more mana without losing more life. Zero talent points in ILT = 15% of your max mana for 15% of your max life, one point = 16.5% mana for 15% life, and two points = 18% for 15%.