As you may or may not know, MariaDB has supported microseconds in version 5.3 for several months already.
And now MySQL has added microsecond support in version 5.6 (5.6.4, specifically).
For more details on MariaDB microseconds, please refer to this page:
And for more details on MySQL microseconds, see “Fractional Seconds Handling” on the following page:
From what I can tell, the implementation looks largely to be the same (at least from a user perspective), in that if the fractional part is given, it must be an integer number 0 to 6. And if no precision is given, the default is 0, for backward compatibility.
MySQL 5.6 is still in the “Development” phase, and MariaDB 5.3 is in the “Beta” phase, so I’d expect MariaDB 5.3 to be GA first, but that’s just a guesstimate on my part.