Because the proc is now magic damage, you can't lifesteal off of it. This makes people who like to combine it with Gunblade sad. ie: Fizz, Akali.
edit: Alright test finished, Lich bane does not proc spellvamp.
I built hextech revolver and lich bane on Darius, took damage from the small golems, then used lich bane on them. No health was gained.