Sterling Quinn of the ArcGIS Server team contributed the following post. This information will ultimately end up in the ArcGIS Server Help site in a few weeks.
ArcGIS Server for the Microsoft® .NET Framework has a scalable architecture that allows you to distribute its components among multiple machines. When you set up a distributed installation, there are a number of post installs, accounts, and operating system groups that you’ll need to configure. These requirements have changed at 9.2, so even if you have configured a distributed installation in the past, you may not be familiar with the latest requirements.
Below is a guide that shows what you’ll need to do on each machine. Each machine in the diagram contains some green text denoting which post install you must run on that machine. Items in blue are accomplished by the post install. Items in red are things that you must do. Note especially that you must manually add the ArcGIS Web services account on each dedicated SOC machine.