i've only experienced this on the 3g iphone. all my phone numbers disappeared from my contacts. the contact info is still there but no phone numbers. hence, when i get incoming/outgoing calls or texts, the number is not tagged to the contact.
my contacts in my computer (mac book pro) are all intact (thank god!) i've tried resynching many times and still the same issue.
I something similar happen recently as well. All the contact data on the iPhone and MobileMe disappeared. I'm not sure which to blame here. The next time I got to my MacPro, I was able to put the proper data back on MobileMe.