In interviews, Kal Penn revealed he originally had a much greater role. It would have been revealed that his character was a disgraced former Daily Planet science reporter who was bribed by Lex Luthor to plant false evidence of Krypton's possible survival, thus inspiring Superman to leave Earth and explore Krypton's ruins.
(at around 40 mins) Superman's line, "It's still the safest way to travel", is a reference to Christopher Reeve's comment in Superman - Der Film (1978) after rescuing Lois from the falling helicopter. "Well, I certainly hope this little incident hasn't put you off flying, miss. Statistically speaking, of course, it's still the safest way to travel."
It is revealed in the video game that Krypton's discovery was a hoax devised by Lex Luthor to draw Superman away from Earth so Lex could be released from prison (for example, when Superman was called to testify, he was a no-show).
The last line of Superman IV - Die Welt am Abgrund (1987) is Superman saying to Luthor, "See you in twenty." That scene was filmed in 1986. Coincidentally, twenty years later, this film was released.
Jack Larson: (at around 26 mins) Jimmy Olsen from Superman - Retter in der Not (1952) appears as a bartender who serves to Clark and Jimmy.
Michael Dougherty, Dan Harris: (at around 57 mins) the two writers of the screenplay make an appearance as school kids making notes as Lex Luthor arrives to steal the meteorite and shoos them away.
Guy Hendrix Dyas: (at around 1h 40 mins) Production designer: one of the journalists of Daily Planet, who first notices the tremor at the building.