SQL Server 2005 SP2 introduces the Vardecimal storage format. The Vardecimal storage format is a new storage format to store decimal/numeric data. This new storage format is not understood by SQL Server 2005 or SQL Server 2005/SP1. Just like you cannot attach a SQL Server 2005 database to SQL Server 2000, attaching/restoring a SQL Server 2005/SP2 database that has been enabled for Vardecimal storage format to earlier versions of SQL Server 2005 will fail. SQL Sever implements this by incrementing the database version number when the database is enabled for Vardecimal storage format. When you disable Vardecimal storage format on a database, its database version is decremented so that the database can now be attached to earlier versions of SQL Server 2005.
Read more about this and what the implications are here: http://blogs.msdn.com/sqlserverstorageengine/archive/2006/12/29/vardecimal-storage-format-and-its-implications-on-backup-recovery.aspx