I have been a huge fan of MySQL for a long time. It is the perfect database for when the budget is tight or you are not working in a Microsoft Environment. It performs well, and has a huge following of dedicated professional programmers that use it day in and day out on some of the largest websites on the planet. Most noteable Facebook, Twitter, Flickr, and Digg. Even with all these proven capabilities to scale and perform, Microsoft has choosen to ignore it and focus on some of the monolytic providers of databases such as IBM and Oracle when comparing SQL Server.
However that has all changed with the release of Microsoft SQL Server 2008. Microsoft has set its focus on MySQL. This is a huge turning point for both companies, because it means Microsoft is starting to take the needs of the Web 2.0 crowd, which MySQL has dominated, just as seriously as the big iron installs they have always catered to.
I am not sure if this comparison has been spured by the purchase of MySQL by Sun Microsystems, or if Microsoft has started to feel the preasure from Web 2.0 MySQL installs, or a little of both. But none-the-less this is very encouraging, because it means that Microsoft is finally taking the needs of the "cloud developers" seriously.