The picture on the wall of the deceased fireman when Shawn & Gus go into the fire station is Director Tim Matheson.
The request that Shawn has to fill out to access information on the buildings being burned is form IG-88, which is the name of the tall robotic bounty hunter in Star Wars: Episode V - The Empire Strikes Back (1980). Gus also says "I have a bad feeling about this", a line that appears in all the Star Wars films.
Shawn tells Milena Govich that her "I told you so face" looks a lot like her watching reruns of Law & Order (1990) face. She had been a regular on that show's seventeenth season.
Spot the pineapple: you can see on Chief Vick's left corner of her desk a little pineapple, next to a pile of documents.
When Shawn is coming up with names to identify their Arson Murderer, he calls the Arsonist 'Señor Pantalones de Fuego.' This is Spanish for 'Mr. Firepants' or, perhaps cheekily, 'Mr. Pants-on-fire'.