The main advantage is that SQL Server (2000 and 2005) are supported out of the box.  DNN used to support MS-Access some time ago but that was dropped when MS announced that you could run production sites on the free version of SQL Server.  Which is the bottom line of this discussion.  You can get SQL Server 2005 Express for free and legally use it to run production sites.  It has some limits but they are pretty high and if you run into these limitations it means that you are running a pretty serious site and you should be able to afford the full version of SQL Server.  Note that the SQL Server Express 2005 database engine itself is not limited in any way (aside form the max memory limit).

There is however support for Oracle and MySQL, you can buy the providers in Snowcovered, yes, you would have to buy those providers.  You can also read about the effort to include the database provider for Firebird here: http://www.dotnetnuke.com/DotNetNukeProjects/ProviderFirebird/tabid/909/EntryID/1284/Default.aspx.

Carlos

I am aftraid is not THAT easy.  SQL 2005 Express has advertised some deployment features like "embedded" deployment and "ClickOnce" deployment but those are just mechanisms to get the SQL Engine software installed with help from Visual Studio.  The reality is that the SQL Server 2005 Express software must be separately installed prior to the installation of DNN.  The thing is that SQL 2005 Express now supports the deployment of the database itself via XCopy, which is basically what DNN is doing..  But it is important to emphasize that this is only for the database file, the SQL Server Express engine software must be installed by hand, so no, it is not like deploying an Access file.  This new way of deploying databases is via a feature called "User Instances".  And by the way, when you go to SQL Server Management Studio Express you will not see the database right away, because is on a User Instance, not the main instance (probably called SQLEXPRESS).  See this article on how to connect SQL Management Express to a User Instance:  http://blogs.msdn.com/sqlexpress/archive/2006/11/22/connecting-to-sql-express-user-instances-in-management-studio.aspx

This is all very confusing, even for experienced SQL Server DBAs (or specially for them) because things work differently.  But you can attach the database to the main instance of SQL Express if you like to work that way.  The process to do it is here:  http://www.dotnetnuke.com/Community/BlogsDotNetNuke/tabid/825/EntryID/1202/Default.aspx

And next is a MS Technet comparison of SQL 2005 Express and MSDE 2000: http://www.microsoft.com/technet/prodtechnol/sql/2005/msde2sqlexpress.mspx

I know, it is a lot of reading and it is all confusing but you asked.   This could also be better explained in the installation document I have to say.

Carlos