Someone has discovered a security breach in iOS 5.0.1 that allows a person to access an iPhone that is has a lock code on it. It takes some work and removal of the SIM card.
It takes a bit of work, but you can watch the video here. Apple is likely to ensure that this hole is plugged in a future iOS 5 update.
As with any operating system there are bound to be security flaws. The good news is that some of these are discovered and attention is brought to Apple in some way shape or form. Even if it is a YouTube video showing how it can be done.