Had a problem with a mixed Exchange 5.5/2003 system where some Exchange 5.5 directory entries were refusing to synch into the Active Directory. Turns out the 5.5 directory entries were pointing to non-existent AD entries. I had to remove the ADC-GLOBAL-NAMES attributes by running Exchange 5.5 Administrator in raw mode (c:\exchsrvr\bin\admin.exe /r). Having done this, a full sync of the ADC got everything working again.
More details in Micrsoft support article 319474.