Boone pointed me to BackupPC: Open Source Backup to disk. Does windows and linux and claims to be a 'high performance, enterprise-grade" system. Anyone tried this?
I've been using it home for about 6-12 months to back up 2 Windows XP computers to a Linux server. Works great!
It's pretty fast, and uses a bunch of tricks to minimize disk space required. Eg, compressed backups, and md5sum checks to check for common files - such as common DLLs & EXEs across multiple Win32 systems.