That question and also does the damage of holy smite increase as you get higher levels?.

Warby

i think liches and skeleton warriors are immune to turn undead at all levels.

also the damage of holy smite does go up, but then so does the magic resistance of the targets, so it gets negated. for me it never really became worthwhile, but i keep one around in case of unexpcected low-level skeleton hordes like in the undercity at the end of BG1.

Liches and skeleton warriors are not immune to turn undead. I can confirm that no undead creature that i have encountered can stand against a level 20 turn undead and not being turned to dust. Allthough maybe a demi-lich might.

JR

Damned i lost cousin Jan's techno gloves

I have witnessed Anomen exploding liches and Skeleton Warriors with Turn Undead - but even at 20th level he has NOT been able to destroy a Crimson Death, though he easily blows up Mist Horrors. (I assume that a Crimson Death is undead.)