The Situation:

I'm attempting to move a database in one server to a different, source server has SQL Server 2008 R2, destination server has SQL Server 2008.

I replicated the .mdf file only I attempted to make use of query sp_attach_db and wizard attach however the was a mistake, the following:

Attach database unsuccessful for Server 'SERVER_NAME'. (Microsoft.SqlServer.Smo)

----------------------- More Information:
The best happened while performing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

The database 'DB_Name' can't be opened up since it is version 661. This server supports version 655 and earlier. A downgrade path isn't supported. Couldn't open new database 'DB_Name'. CREATE DATABASE is aborted. (Microsoft SQL Server, Error: 948)

I dont think its likely to add a database from the greater version of SQL server to some lower version.

Eg 2008 to 2003 isn't ok,

going from 2003 to 2008 is alright.

You could attempt doing an export import data however i haven't exercised how you can attach it to date.