From time to time I have encountered various synch problems between my various Macs, iPhone, and iPod Touch, but I have never had any problem with data being stripped from an AddressBook entry. Can you be more specific about what is happening:
  1. Is the data being stripped found in custom fields, standard AddressBook dat fields, or both?
  2. By any chance is this data in the Notes: field?
  3. Where are you first noticing the missing data is it..
    1. On your Mac
    2. On your iPhone
    3. on the MobileMe contacts list
  4. When you notice missing data on one device, do you ever immediately check the other two devices to see if the data is missing on either or both of them?
Obviously this is preliminary troubleshooting and at this point, I am looking for clues. I have more questions, but which questions would be based on your answer to these.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein