All Enhanced Edition infinity engine games and their expansions are based on AD&D second edition rules, yes. This also includes the upcoming Siege of Dragonspear.
my main issues with 2nd ed was ac is better the lower it is which is the opposite of every rpg i have ever played and throws me off all the time. and the magic system but the patch may fix the one.
While the game is certainly based on AD&D there are definitely some differences. Weapon proficiencies have been changed as one of the examples. There are better webpages if you want to know the more specific stuff.
Comments
http://cdn.baldursgate.com/files/baldurs-gate-mastering-melee-magic.pdf
http://www.dmsguild.com/product/16868/Players-Handbook-Revised-2e?it=1&filters=45471_0_45381_0_45347_0