Ratchet & Clank: Up Your Arsenal Tips

Multiplayer cheat!
In the three levels where you can get a plane, this can be used. Get the plane and bring it right down to the water. Then get out of the plane, and as you are sinking in the goo, get back into the plane. You can now walk through walls.

*addition by Link182: This used to work before, but if you have a patched version of the game (updated) it will not work.