You experience that Data Synchronization fails when a DATETIME column is used for the Primary Key (alone or in conjunction with other columns). You probably get the 'infamous' MySQL Server error 1064 'You have an error in your SQL syntax ....'.
MySQL AB has confirmed a change with the most recent MySQL builds (4.1 higher than or equal to 4.1.13 and 5.0 higher than or equal to 5.0.8 (and most likely 5.1.x as well)). It is described here http://bugs.mysql.com/bug.php?id=16546 .
Executing/calculating 'datetime_variable+0' is exactly what the DATA SYNC tool does to convert the DATETIME to a number.
At the time this functional change was introduced there were no noting of it in the MySQL changelogs, so we did not (and did not have any chance to) change the code of SJA accordingly.
Starting from SQLyog 5.16 we TRUNCATE (datetime_column + 0, 0)' for SJA to be able to work correctly with DATETIME-based PK's on all MySQL versions.