In response to a post I just read (http://blogs.conchango.com/jamiethomson/archive/2006/10/18/VSTS4DBP_3A00_-CTP6-is-live.aspx) from Jamie Thomson I thought I should post a little more information regarding the teams decision to support SQL Server Express.
As Jamie points out, Gert Drapers post announcing CTP 6 did state that, moving forward, Data Dude would no longer use or support SQL Server Express as its underlying database. Jamie's concerns were around the fact that this might imply an additional cost as you are now required to have SQL Server Developer installed.
To clear this up, the decision was made to stop using SQL Express and to move to SQL Server 2005 Developer Edition in order to support the full range of enterprise features supported by SQL Server Enterprise (SQL Server Developer is essentially Enterprise Edition licensed only to be use for development and testing). SQL Server Express is a desktop database and does not support all the Enterprise functionality supported by Developer Edition. By using the Developer Edition rather than Express the team is able to supply support for all the functionality of SQL Server without requiring you to install another database to develop enterprise level functionality
There will be no charge for developer edition and it will ship in the box with every copy of Data Dude so this will not affect the cost of using DataDude.