Stephanie Pratt

Stephanie Pratt spent Saturday evening at a birthday party in Hollywood for Holly Montag. She spent Sunday morning in jail after she was arrested for DUI. A good time was had by all. Maybe too good. Stephanie Pratt Busted for DUI

UPDATE -- A source close to Pratt tells us Stephanie had just left Holly Montag's birthday party when she was pulled over. Our source says cops watched Stephanie get in her car and were waiting to bust her. TMZ has learned Stephanie Pratt was arrested for suspicion of driving under the influence…