Microsoft is once again changing licensing schemes following leery admins disliking their draconian restrictions on the number of users and the databases used in Windows Server 2008. With Apache still being the overwhelmingly popular choice for web servers today and Linux/MySQL/PHP being a combination more popular than IIS, Microsoft thinks that lessening their restrictions will help them steal more customers away from these platforms.
Their changes will allow any type of database software to run without restrictions on the number of users accessing in, provided it is an internet-facing front-end server. Other changes to the licensing scheme also seek to make it more appealing, though you can't help but wonder what other hidden catches are buried deep within Microsoft's EULA.
Microsoft has been gaining ground against Apache, very slowly, and they obviously want that trend to continue.