Shadow copies of shared folders are point-in-time snapshots of a shared folder that are exposed to Windows 2000+ users by way of the "Previous Versions" tab on a shared file.
The shadow copies client is built into Windows 2003 and Windows XP SP1 and above, and available for download for Windows 2000.
Yes, it is supported in Samba by way of a Samba VFS module.
The only source of documentation on setting up Samba to provide shadow copies to Microsoft clients is the Stackable VFS modules section of the Samba HOWTO.
This document will give you a good idea of how to set this all up.
This document is written using Ubuntu Dapper, but you should be able to use the information on any distribution with new enough components.
LVM provides a method for taking a snapshot of a volume; it is these snapshots that will be shared and provided to Windows clients as shadow copies. XFS supports freezing a file system, buffering writes to it until it is thawed again. To be sure of the integrity of your snapshot, you need to do this. It seems like it's possible to do this on Ext3 file systems, but without a similar method for freezing, you could lose data.
One page links to SambaShadowCopyHowto:
