The new Report Designer in SQL Server 2008 is actually a combination these two products from SQL Server 2005:

• Report Builder - A stand-alone application distributed through SQL Reporting Services that is pretty high-level and designed for end-user ad-hoc reporting.  The files it generated were the standard .rdl files ... however, there were many limitations this software like the ability to only have one report region (couldn't make a single report that contained a few different types of information or queries) and you could only pull from one data source (typically a single report model).
• Report Designer - This was a tool that developers and other IT pros could use to design more specialized or complicated reports inside Visual Studio.  The interface was less than intuitive, but it didn't have those limitations found in Report Builder.  It also saved files in .rdl format, but if you created a report with Report Designer ... Report Builder would not be able to edit that report.  However, if the report was created in Report Builder, you could always open it with Report Designer and modify it there.

So now there is just one product ... SQL Server 2008 Report Designer.  It looks more like the stand-alone Report Builder, but has the Office 2007 look and feel including the ribbon control.  This tool is supposed to be a common platform that is easy enough to use so end-users (e.g. management, executives) could create ad-hoc reporting, but also contain all of the functionality needed by IT pros to build more complicated reports as well.  Seems like a tall bill to fill, but it isn't improbable in theory ... so maybe they got it right.

The link below is a very recent "web exclusive" article from SQL Server Magazine that gives fairly in-depth preview on the product, and some examples and screenshots.  Check it out:

http://www.sqlmag.com/Articles/Index.cfm?ArticleID=98830