Receiving Phone Call-Nothing Displayed iPhone

Hello!
Ive had a strange problem recently with my iPhone 3GS. I will receive a call (noted by the ringtone), but when I look at my phone to see who is calling, nothing will show up. The top bar that would normally have the contact name or phone number has the grey overlay, but is blank. The bottom allows me to slide to answer the call. People are still able to leave messages, and at that point I am able to see the number the person called from.
Whats happening?
iPhone 3GS - ATT iOS 4 iOS 4.3.1