Welcome to 3.0. Mana spring has stacked since before WoTLK. It changed from a noticeable buff to a "pulse" like tremor totem, as did healing stream totem. So yes, mana springs stack as much as you want Too bad they dont get a boost from +spellpower like healing stream does or 1 mana spring would probably be enough for infinite mana (of course thats probably why they didn't do it LOL).