SharePoint Engineers at Aivea has been running SQL 2008 for serveral months, yes x64 bit on Windows 2008 as well. Aivea SharePoint test labs now have about 4 physical machines and 7 VMs on MOSS 2007 - Windows 2008 - SQL Server 2008. Here are the top 8 reasons for using SQL Server 2008 for MOSS 2007 implementations

1. No more 2 Gig per File Limit, SQL 2008 has 8 Gig per File and you can stream out the BLOG objects. No need to wait for ClouldSpace as well
2. Backup compression (25-35% at least) - huge cost savings and hey it's native!
3. Transparent Encryption - transparently encrypt the data without the application needing to be aware or know about it.
4. Database Mirroring Enhancements - compressed tlogs, more automatic failover options
5. Policy Based Administration - manage policies across databases and even across instances and servers
6. Cluster enhancements - easier to configure and more scenarios that it supports including many in WS08 that provide more intelligence for failover
7. Resource Governor - limit wild database resources or control one database resources from another!
8. Reporting Services in SharePoint Mode (report builder rocks!) - easy reporting solutions built to leverage SharePoint mode with more flexibility and with ease.