Is it a good idea to include the writing of a wikipedia article(s) on your CV before grad school application?
The notion of authorship in wikipedia is rather antithetical to the notion of authorship in "standard" academic works.  Wikipedia articles don't have "bylines," and the point is that anyone in the world is welcome to make constructive edits and/or additions to an article at any time.  Even if you feel that you are the primary or sole author of an article at time t_1, at some time t_2 > t_1 this may no longer be true. Finally, the ethos of wikipedia is such that other wikipedians would not necessarily be happy to see a given person claiming "credit" in this way.  

If you are motivated to write such a wikipedia article, I suggest that you give it a go -- it has the potential to be both a learning experience for you and result in something of interest and use to many people.  However, if you want academic credit I suggest you write something more substantial, formal and/or independent.  As the head of a graduate admissions committee (albeit in mathematics, not physics) I would be more impressed even by a sequence of blog posts than by a wikipedia article.  Still probably not crucially impressed by the way, but more impressed.