Title says it all, but is this really a feature you can benefit from or is it some piece of random engineering that the dev team decided to do because it would be "fun"?
One of the big challenges with SSRS has always been config, in fact I would argue its a problem for all of SQL Server 2005 so much so that we added a completely new tool to help manage it (technically it was 2, SQL Config Manager and Surface Area Config but SQl-CM really replaced 3 smaller utils that had been around for years). In general SQL 2008 is moving to a cleaner config model (and the separation of setup into copy and config).
Config simplification is one of the reasons for the move to not use IIS, but the other is around memory, thread and scale management, if you have every tried to generate a 100 page PDF report you will know what I mean.
Brian has a great article describing the reasons behind the change in detail, my investigation on a 40 page report with CTP-5 shows a decent improvement in rendering time, but with a greatly reduced memory load during rendering. While in VSTS we don't tend to generate a lot of multi-page reports we do have a lot of concurrent users and the reports can be pretty memory intensive.
Looking forward to the RTM bits.