This is the first time that Leonard Nimoy has been cast in the series. The producers of the show have been trying for many years to get Nimoy to appear on the show, which was difficult due to him having mostly retired from acting.
When Sheldon dreams that he's woken up on an alien planet, the distinctive arch of the Guardian of Forever from the original Star Trek series episode "City on the Edge of Forever" is visible in the background.
Raj's date Lakshmi is probably the same Lakshmi who was "flying in from London for the sole purpose of meeting him" and intended as his date for cousin Sanjay's wedding mentioned in The Guitarist Amplification (2009).
Penny's residual check for an old commercial is most likely the hemorrhoid cream commercial she did in The Skank Reflex Analysis (2011).