How to move SQL Server databases to a new location by using Detach and Attach functions in SQL Server
This is probably the most straight-forward explanation I've come across on how to do this.
I used it to move my 5GB Thumbsplus database from my raptor that was running out of space to a larger drive and I followed the directions in the article and it pretty much went smooth except I got a permissions error of some sort which I easily fixed by running SQL Server Management studio as administrator :)
Also, I used SQL2008 but this article only applies to SQL2005 and older it looks like.
You are supposed to look at SQL Server Books Online for 2008 but that didn't help me any.