I've shown a few members how to do this, and it was suggested that I share this here...
If you've ever had someone grab your phone, and had a worry about what they might stumble upon in your web history (or currently open windows), read on...
Essentially, all you need to do is use a different browser for your "naughty" browsing activities. The best one I have found is called "Atomic Web" and is available in the Apple app store.
But don't download the free version, the paid version is only a buck or two and allows passcode security, allowing another level of protection from prying eyes. If you want to take it further, hide it in a discreetly named folder and your habits should be well hidden.
The app employs tabbed browsing, and a few other nifty features that put it above the standard Safari app. More info may be found here: http://www.pcworld.com/article/194488/atomic_web_a_better_ipad_browser.html
I only use an iPhone (it's also available on the iPad), but I assume the app (or something with a similar security feature) is also available in other mobile platforms.
If you're looking to hide photos, another great app is "Private Pics," which also has passcode security. You can create albums and easily add photos... but don't forget to delete them from the standard Photos application. It makes a copy but does not automatically delete the original.
Hope this helps some people. Happy browsing!