'rro - In Johnny the Homicidal Maniac, Johnny's nickname is 'nny.
seeing the lines - In the dating sim game and anime Shingetsutan Tsukihime, the main character Shiki sees lines everywhere. The lines belong to objects, and if he cuts a line, its object is destroyed. Shiki uses a switchblade knife much like the one Piro has in the last panel.
Bonus pun! Piro+Shiki = Piroshiki (or Piroshki), from which Kanon derived the name Piro in the first place.
True to Grand Theft Auto: Vice City, the police are just as corrupt as the criminals.
Squee... - Besides being a drunken slurred squeek, Squee is the name of a character in Johnny the Homicidal Maniac.
They say things like I'm not funny anymore, or that I'm too mushy - Common complaints from MegaTokyo readers who miss the more random humor of the very early strips.
Kitty ears - The sudden appearance of kitty ears is reminiscent of the third episode of the anime FLCL. They may also be a reference to the cat Piro in Kanon, whose name Fred Gallagher is forever linked to as a result of MegaTokyo.
P? - Parody of the Z? shirt of Johnny the Homicidal Maniac.