There are messages from the developers hidden on top of the jungle temple. To see them you need to be in an alternate rendering mode.
According to a Gamasutra article for Cel Damage, it was initially known as Cartoon Mayhem and was designed as a racing game with cartoon gags and elements thrown in, and was intended to be published by Microsoft exclusively for the Xbox. Creative differences between Psuedo Interactive and Microsoft led to the former signing a deal with Electronic Arts.
A sequel to Cel Damage, Cel Damage 2, was intended to be released in late 2003 and featured a wealth of improvements, including new gameplay mechanics where the player can go on foot and hijack enemy cars. Sadly, it was cancelled due to the original game's lack of profitability and the lack of any interested publishers.