When Howard sarcastically chants, "One of us, one of us" after Leonard says, "You're officially one of us!" this is a homage to the 1932 black-and-white film Freaks where deformed humans are joyfully chanting for the female protagonist Cleopatra (whom Penny is being compared to) "Gooble gobble! We accept her! One of us! One of us!"
When Leonard gives Penny the number for Sheldon's mother, he paraphrases Truman's secretary of war, Henry L. Stinson, who justified the use of the Atomic Bomb by stating that it will shorten the war by five years and save millions of lives.
The trivia item below may give away important plot points.
Analeigh Tipton is an American, model and actress. She has appeared twice on TBBT in season two "The Panty Piñata Polarization" as herself as one of the models that Howard and Raj meet at the house used in "America's Top Model" and in season nine "The Mystery Date Observation" as Vanessa Bennet, Sheldon's potential date.