Near the end of Act 3, Mission 7 "Phantoms", when you go up the lift, you will see enemy robots dancing with an enemy barrier speaker.
According to the game's creative director, Shinji Mikami, the staff hid rhinoceros beetles in the game without him knowing.
There were plans to make a special edition that featured an alternate black suit. However during production the plan fell through.
A lot of the game was inspired by the anime Casshern, with a heavy emphasis on speed for gameplay. The background visuals were initially influenced by the movie Tristan + Isolde, but the staff ended up using a personal style.
There was originally going to be a robot dog companion that assisted in combat and combined with Sam's armor. The concept was eventually dropped.
There is an achievement/trophy called "Fisher is the other Sam" which can be obtained by proceeding on the monorail without being spotted by the enemy troops or searchlights. This is a reference to Sam Fisher in the Splinter Cell series in which stealth was the primary game-play element.