The Monopolar Expedition
The Big Bang Theory Season 2 Episode 23 (05/11/09)
When Sheldon finds out he's getting an opportunity to go to the North Pole for three months on an NSF expedition, he struggles with the choice. It is a chance of a lifetime to help prove string theory... but it involves something he hates, going outside. The gang also seems extremely excited to have a Sheldon-less summer. That is until Sheldon decides to go and asks the guys to be the members of his team. The boys decide it'll be worth putting up with Sheldon for three months to help make scientific history.
When Sheldon asks Penny to use the meat locker at the Cheesecake factory in order for them to practice, Penny is hurt to find out that Leonard didn't tell her about the trip. Penny gives Leonard a goodbye gift (a snuggie) and an elongated hug (5 Mississippi count!). Leonard is convinced Penny still likes him and goes to say goodbye to her the morning before they leave and Penny won't admit her feelings and how much she'll miss him to his face.
The guys end up going to the North Pole and we see them eating reconstituted Thai food and watching movies. Sheldon still has his spot on the couch. Guess things aren't so different there.