В общем-то, раньше это было невозможно, Application существовало в каждой ветке дерева, со своим global.asa, не то что в отдельных сайтах
Я могу ошибаться, не сталкивался с этим, но было бы интересно узнать, -кажется в IIS 6 application pool позволяют иметь общий контекст для Application, в него входящих?

вот я тут нашет цитату

Working with Application Pools
Application pools set boundaries for applications and define the configuration settings that applications they contain use. Every application pool has a set of one or more worker processes assigned to it. These worker processes define the memory space that applications use. By assigning an application to a particular application pool, you’re specifying that the application:

Can run in the same context as other applications in the application pool All applications in a particular application pool use the same worker process or processes, and these worker processes define the isolation boundaries. These applications must use the same version of ASP.NET. If applications in the same application pool use different versions of ASP.NET, errors will occur and the worker processes might not run.

Should use the application pool configuration settings Configuration settings are applied to all applications assigned to a particular application pool. These settings control recycling of worker processes, failure detection and recovery, CPU monitoring, and much more. Application pool settings should be optimized to work with all applications they contain.

The sections that follow provide techniques for creating, configuring, and optimizing application pools.

Microsoft IIS 6.0: Administrator's Pocket Consultant
by William R. Stanek ISBN:0735615608
Microsoft Press © 2003 (449 pages)