i just wanted to say thanks to Matthias too!! 😀
i had a bit of an experiment as well.... i am using an iphone 4. i updated from 4.3.3 to 5.0.1. i had the issue of the 'contacts, mail and calendar' options crashing (back to home screen), could not add contacts (crash to home screen), typing a name in sms would not bring up a list of possible contact matches, etc....
i reset to a clean 5.0.1 and everything worked... the moment i restored my phone in anyway (from both icloud and itunes) the problem returned. if you install clean and then DO NOT USE OLDER FIRMWARE BACKUP RESTORE AT ALL, it works fine from there on. you can then create a backup of your newly installed stuff and restore with icloud etc, and all works perfectly. 🙂
so for me, something from my older backups was effecting ios5. do not use your old backups. start, and stay, fresh with 5/5.0.1. 😉