You can try going to settings > general > reset and try resetting the keyboard dictionary.
There is also an option to erase content and settings.
make sure that your contacts and calendars are sync'd with the computer before you use erase all content and settings.
If that does not work, you can always restore the iphone and set up as new iphone.
If you restore from backup, you will put the auto fill information back on the iphone.
If you decide to restore the iphone, this article from the apple support website might help out.
http://docs.info.apple.com/article.html?artnum=305744
Hope this helps.