do you just need to hide it from the people who may browse on your phone? Or really need security if someone stole it.
If it's just from friends playing with it, hide pictures and such, there is one. It's called "Hide N' Seek, and to the untrained eye, its just a flashlight app. But if you press menu 3 times, and then enter in your corresponding password, you can "hide" files. However all it really does is add ".hide" to the extention, so its no longer a .jpg or .mov, etc. So your programs wont pic it up.
It costs $1 and is on the market. There may be a free version, I'm not sure.