My personal expreience says that you wont have any migration issues if you have sql server 2005 db instances. The attach and detach of DB is same in 2008 version. Only thing is, once you use a 2005 instance DB and attach it to 2008 version, it cannot be used with 2005 version again. Apart from that, from functionality point of view, there are no issues, you can continue to use the 2005 version DB on 2008 as normal.
link for moving the DB from 2005 to 2008 -
http://www.packtpub.com/article/moving-a-database-from-sql-server-2005-to-sql-server-2008-in-three-steps
EDIT: Incase you want to move back to 2005 version for same DB instance, I dont think it will be possible except for doing a data export import onto new 2005 DB instance.