I have had my iPhone 4 since summer, and am starting to have a couple peculiar problems with it.
The first one - when I start text messaging and open a thread, about half the time the keyboard does not appear. The only thing showing is the list of messages and a big blank area where the keyboard ought to be. I have to go back to the list of threads and reopen it to get the keyboard. Thats annoying and recent behavior.
Also recent is the unread message badge and indicator showing me there is an unread message when there isn't. The only way to clear that is to turn the phone off and back on.