The gas Leonard brings to Penny is Sulfur Hexafluoride. It is 6 times denser than air, thus causing the 'Darth Vader' voice. It is an inert gas and safe to breathe in small quantities.
In the opening scene, Howard gets a phone call from "Dave Roeger at NASA". Dave Roeger is a name of another computer programmer who worked with Bill Prady back in the 1980's.
Casey Sander also appeared on Sons of Anarchy (2008) in episodes 3.6 The Push (2010) & 3.12 June Wedding (2010) as a cop.