There are a number of reasons to rather do an OFF LINE rather than a DETACH when moving database files like this. In SQL 2005 there was a particular issue with permissions being lost, but it looks like this happens ocassionally in SQL 2008 as well. I've never seen this happening using OFF LINE.

Another issue is a detach losses some database spesific configurations that are maintained in the MASTER database (e.g. cross database ownership chaining - not a recommended option), which aren't lost when an OFF LINE is used.

Cheers

Leo