Last week i had the chance to try migrating SharePoint to 64-bit environment running on SQL 2008.
I made a clean installation on a single machine,SQL Server 2008 + SharePoint 2007 and then applied a "stsadm -o restore..." command to restore my SharePoint which is running on 32 bit environment.
Worked fine! but i did not try to migrate the custom applications yet. they all have their own content db; it will be another challenge.