Heidi Harley has noted the presence of Steve Kowit's poem "The Grammar Lesson" on the Poetry 180 site. This is a nice poem, but my favorite poem with a grammar theme is Kenneth Koch's "Permanently". This is the one that opens,

One day the Nouns were clustered in the street. An Adjective walked by, with her dark beauty. The Nouns were struck, moved, changed. The next day a Verb drove up, and created the Sentence.

It was a reading at my wedding, which I suppose tells you something about how much of a grammar geek I am.