The November CTP of SQL Express SP2 is available at a download center near you. As I've written before, SQL Express SP1 has a few known issues when running on Windows Vista, SP2 is where we've addressed these issues and is what we recommend using for Vista compatibility in your applications. The CTP is available from the Microsoft Download Center:
SQL Express SP2 CTP
SQL Express with Advanced Services SP2 CTP
SQL Express Toolkit SP2 CTP
These CTPs are not for production distribution, but you should feel free to start testing your applications. We will be posting SQL Express SP2 to Microsoft Update (read my post about that here), and the CTP is your chance to validate this release with your applications and ensure they work before you customers apply it in the wild.
Feel free to file any issues you find in the SQL Server Feedback Center.