SP2 brings with it enhanced support for the "branch office scenario" - by this we mean reducing the cost of communications between remote sites and their head offices.
SP2 brings support for HTTP compression, diffserv (traffic prioritisation) and Background Intelligent Transfer Service (BITS).
HTTP compression is a feature of many web servers including IIS v6 & 7. Outlook Web Access traffic for example can be compressed leading to better performance. With SP2 ISA can be used to cache compressed HTTP traffic. ISA's HTTP compression conforms with RFC 1951 & 1952.
ISA's BITS caching capability means that it is able to cache patch and security updates from Windows Update.
Diffserv support means that ISA can be configured to respect Quality of Service (QoS) attributes in IP packets and to prioritise traffic accordingly.
ISA SP2 combined with Release 2 of Windows Server 2003 which adds Remote Differential Compression (RDC) should lead to significant bandwidth savings. You can find out more about Windows Server R2 by clicking here.
RDC replication is the replacement to the File Replication Service you can read more here. The prime benefit of RDC over FRS is that only changes to files are sent over the network so if you change part of a file then only the updates are transmitted - you can even change the filename!