Hi,
Migration of data is always tricky, yet as already said in this thread, Windows architecture doesn't allow accessing the same LUN from two different hosts at any given time (with the exception of Hyper-V, but that's completely different story).
Personally I'd detach databases from the old server, disconnect LUNs, mount them to the new one & simply upgrade databases from 2005 to 2008 (via SQL Management Studio).
Needless to say, doing backup upfront is strongly recommended! (as usual )
Regards,
Radek