While a similar bug in iOS 4 was reportedly squashed, iOS 5.0.1 is also not without flaws. According to a recent video posted on YouTube (at bottom), it is possible to bypass the iPhone lockscreen and enter the Phone app where you can make phone calls, access recent contact and even get on FaceTime.
This hack reportedly only works on GSM based iPhone models so Sprint and Verizon iPhone users may be safe. The method involves trying to return a missed call while the SIM card is taken out and re-inserted repeatedly (or in an area with no network coverage).
This will eventually work and unlock the Phone app, after which the user is free to re-insert the SIM card and get back on the network.
Now a similar bug was found in iOS 4 where Smart Covers would allow the unlocking of iPad screens. There was also another bug in iOS 5 which allowed access to the Camera Roll, although to replicate it was no easy feat.