SQL Server 2008 Express is now available as part of the November CTP - You can download it from the following link: http://www.microsoft.com/downloads/details.aspx?FamilyID=749bd760-f404-4d45-9ac0-d7f1b3ed1053&displaylang=en.
There is already a bunch of information out there about the new functionality available in SQL Server 2008, so I won't spend much time describing the features here. What I will do is document which of those new features will be available in the Express edition.
The following new features are available in SQL Express 2008:
DB Engine Improvements
For the November CTP we have only released the basic version of SQL Server 2008 Express. We'll be releasing SQL Server 2008 Express with Advanced Services and Management Studio Express in a future CTP. For now, if you want to use management studio to work with your SQL Express database, you should download SQL Developer edition, which is available from the same site listed above, and install the full version of Management Studio. This will work against SQL Express with no problems.
Before you even ask, yes, the download package is pretty big; hey, it's a CTP! The download size will be reduced as we approach the final release so don't get too worried at this point, it's an interim step.
You can provide feedback (bugs and feature request) on the SQL Server 2008 November CTP through the SQL Connect site: https://connect.microsoft.com/SQLServer
You can post general SQL Server 2008 question in the appropriate SQL Server 2008 forum on MSDN: http://forums.microsoft.com/MSDN/default.aspx?ForumGroupID=428&SiteID=1
You can post SQL Express specific quesitons in the SQL Server 2008 Express forum on MSDN: http://forums.microsoft.com/MSDN/ShowForum.aspx?ForumID=1621&SiteID=1