Ran into a problem today: our development is currently on a server running SQL Server 2008 yet our live environment is running SQL Server 2005. Migrating from earlier SQL Server 2005 to SQL Server 2008 is easy, backup a database (create a bak) and restore on the new database. Even detaching and reattaching works the same way. But not the other way round. I have even tried overwriting the MDF and LDF files, yet with no success. Kept on getting an error about the incompatibility of the versions of the SQL data headers. Till I found this post: restoring or attaching databases from sql 2008 to previous versions. Very much recommended.
The only thing the author doesn’t cover is how to transfer the data with the table structure. It isn’t that hard, let me know if you struggle 
Good luck!
