Does Humility break Grimgrin?
It will untap.
The fact that it enters the battlefield tapped and that it doesn't untap normally are put into one static ability for space-saving purposes (compare with Leviathan which also has lots of other text, and with Deep-Slumber Titan, which doesn't). This doesn't make the “doesn't untap” clause part of the replacement effect, the ability has to exist at the time you would untap it for it to function.
As Grimgrin enters the battlefield, you check for replacement effects replacing this event. Humility isn't active yet since we are treating these replacement effects before it enters the battlefield. Its entering the battlefield is replaced by “enters the battlefield tapped”. That's all the replacement does.
During your untap step, you check whether Grimgrin has any abilities preventing it from untapping. Because of Humility, it doesn't, so you simply untap it.
Daniel Kitachewsky
L3, Paris, France
Rules NetRep