As a Cheesehead (born in Wisconsin, near Milwaukee) I've seen that spelling often.
I was really surprised that it wasn't in Webster's Dictionary!
It is in the Oxford Dictionary though. The definition pretty well tracks how I've seen the word used.
simplified spelling of night
grist for a million late-nite TV jokes
"nite". Oxford Dictionaries. April 2010. Oxford Dictionaries. April 2010. Oxford University Press. 11 November 2010 <http://oxforddictionaries.com/view/...1271577>.