Microsoft has a new whitepaper up on MSDN that shows some pretty dramatic performance gains when you use transactional replication between geographically distant Publishers and Subscribers with Windows Server 2008 and SQL Server 2008. Both the Publisher and Subscriber must be running on Windows Server 2008. These gains are mainly due to the rewritten network stack in Windows server 2008.
I have personally seen very dramatic file copy performance improvements when you are copying large files between two servers that are both running Windows Server 2008 (even on Gigabit ethernet in the same data center). A common scenario for this is copying transaction log backup files between a principal and a mirror as you are “preparing the mirror” for database mirroring.
SQL Server 2008 Database Mirroring also has log stream compression turned on by default, which will help out in geo-mirroring scenarios. If you use SQL Server 2008 Backup Compressio, both your full backups and your transaction log backups will be smaller, take less time to backup and restore, and will copy over the network during mirror preparation much more quickly (especially if you use Windows Server 2008).