Can I attach a SQL 2008 R2 database to SQL 2008 that is not R2? Can I attach a SQL 2008 R2 database to SQL 2008 that is not R2?

I have a database that I am detaching from SQL 2008 (not R2) and can attach this to SQL 2008 R2.

Can I then detatch the database from SQL 2008 R2 and attach it to SQL 2008 (not R2)?
No.

You can't do that from a higher to a lower version. R2 is considered as a new version of sql server. You can go from lower version to higher version with detach/attach.