Are all of your contacts showing up correctly in the contacts app of your iPhone, and or on your computer?
Do you have Mobile Me? I have had problems with Mobile Me in the past, and my contacts get corrupted and then when it updates my contacts on my phone I get only numbers. But I have not had it happen where only some of them are lost, when it has happened it has been my entire contacts list. I then must re-sync my contacts from my computer to Mobile Me, then my phone is updated.
If your contacts info is all good on your phone, I'm afraid to say I have no idea as to why this is happening. But if you subscribe to Mobile Me and have it sync to your phone, log in and see if those contacts are there.
Good Luck,
PM