A contact whose last name is Null (not the value, but literally N-u-l-l) will, when exported, show up with the Last Name data field blank.
According to the 1990 US Census, only .003 percent of Americans have the last name of Null (ranking #4509 on the list of last name frequency), but it's common enough that if you were to exclude last names that appeared less frequently, you'd be excluding 38% of the American population.
Plus we've got a Null in our database.
This problem has been replicated on the demo by exporting contacts from a Null Group that was created.
The way CiviCRM handles the Null name seems to extend to other data fields; a Null in the City data field was also exported as blank. (Not that there are any cities or towns named Null, but just to illustrate...)
Lobo replied (on the Forums):
please file an issue for this. We'll attempt to fix this in the 2.1 release (since it only happens for 0.003 % of the names). the string 'null' is being used by the base package that we use (DB_DataObject) in a special manner (i.e. make the field null) and hence the fix is not as trivial as it sounds