This is was originally made by some one else but I added the Max ammo.
Dpad Up turns you invisible
Dpad Down gives you Invicibility and Max Ammo
Back Butto turns off Invisibilty
Might have to press the Dpad Down after each save to turn Invicibility back on.
If I can find more cheats I will update in future.
Make a backup of your ThirdEchelonUser.ini then either ftp this to your hdd if you have splinter cell on your hdd, other wise put this file in your splinter cell back up before you burn it to disk.
Have fun!