This little tid-bit just popped up on the iPhonePOV boards and it's such a doozie, it deserves special notice.
Basically, it boils down to this: if you surf porn on your iPhone, you might get caught!
When the you clear Safari's history by pushing the 'clear history' button under Safari's settings, the iPhone only erases the text-based 'history' of sites you have visited, it does NOT erase all memory of the site from the browser. This means if you push the 'back' arrow on the browser, Safari will load a previous site, even though the history should be cleared.
If you want to truly clear Safari's history of all incriminating evidence, this is the correct and rather tedious procedure:
After pushing the 'clear history' button in settings, go back to Safari and touch the pages button (overlapping squares lower right), click "new page" - a blank page will load full screen. Then push the pages button again and close the previous page(s) by pushing the little red X. The new page has no history.