The SQL Customer Advisory Team just released a Technical Note comparing SQL Server Reporting Services 2008 vs. 2005 from a scale-up perspective.  Its good to see that a lot of the work that we did over this release focusing on performance and scalability (across the board, from the core server/processing infrastructure to specific rendering extensions) has really paid off.  You can read the entire article here:

http://sqlcat.com/technicalnotes/archive/2008/07/09/scaling-up-reporting-services-2008-vs-reporting-services-2005-lessons-learned.aspx

Quoting from the summary (emphasis is mine):

Reporting Services 2008 was able to respond to 3–4 times the total number of users and their requests on the same hardware without HTTP 503 Service Is Unavailable errors compared with Reporting Services 2005, regardless of the type of renderer. In stark contrast, Reporting Services 2005 generated excessive HTTP 503 Service Is Unavailable errors as the number of users and their requests increased, regardless of the report renderer.

Our tests clearly show that the new memory management architecture of the report server enables Reporting Services 2008 to scale very well, particularly on the new four-processor, quad-core processors. With our test workload, Reporting Services 2008 consistently outperformed SQL Server 2005 with the PDF and XLS renderers on the four-processor, quad-core hardware platform (16 cores) both in terms of response time and in terms of total throughput. Furthermore, with these renderers on this hardware platform, Reporting Services dramatically outperformed other hardware platforms regardless of Reporting Services version, responding to 3–5 times the number of requests than when running on either of the other hardware platforms. As a result, we recommend that you scale up to four-processor, quad-core servers for performance and scale out to a two-node deployment for high availability. Thereafter, as demand for more capacity occurs, add more four-processor, quad-core servers.