There is a claim circulating that recent quirky App Store hit Flappy Bird was inspired by a previous game Piou Piou vs Cactus. Flappy Bird's creator Dong Nguyen has reportedly denied this. It's clear that there are some similarities (the big-lipped bird graphic and general game mechanics). On the other hand, there are some distinguishing elements (in Flappy Bird, the player flies between two pipes and must therefore exercise more control, compared to Piou Piou where the player apparently flies under or over a cactus and where they are only killed if they are pushed off the screen by the cacti ).
Give that his game predates Flappy Bird, the author of Piou Piou is understandably frustrated that Apple have refused to admit the game to the App Store on the grounds that it "leverages the top game 'Flappy Bird'". However, I think it can't be denied that releasing it to the App Store precisely now is an attempt to capitalise on Flappy Bird. My advice would probably be for him to take a practical stance, to go along with Apple and try submitting a further modified version (after all, we're not talking about a terribly complicated code base!).
This situation does highlight a point of frustration for developers generally, though. It's difficult to know specifically which elements of gameplay are what Apple see as being too similar. Is it the tap-to-jump dynamics? Is it the fact of having a sideways scroller with only static obstacles? Are bird characters with fat lips outlawed now? What independent game developers are surely taking away from this-- and the general element that many will want to build on-- is that there is a market for mobile games with extremely simple dynamics requiring a short attention span. And like it or not, Flappy Bird is a canonical example of that type of game.
Update: more reports of Apple refusing Flabby bird clones.