One of the most important aspects of the MySQL suite to many of its users has been its open-source nature. For that same reason, Sun's acquisition of MySQL was received with much skepticism, though a few short weeks after the fact, it seemed like it would be business as usual for the database company that has kept working on a new version (5.1).
But now Sun has announced the inevitable, as they will begin to close-source certain components of the MySQL package and making those available to MySQL Enterprise clients.
This constitutes a large and serious change to MySQL development. It's no surprise that the number of companies running MySQL Enterprise represent a very small fraction of the totality of servers relying on MySQL, which could begin putting Sun at odds with all of those users. Ultimately, the more feature-rich MySQL Enterprise may be all but abandoned if too many people cease using MySQL because of this. Time will tell, but this does not bode well for the MySQL community.