I wanted to add to this a handy workaround I found if you're in a particular situation. You can actually use Visual Studio 2005 and create SSRS 2005 reports and still pull from SQL 2008 data. This can be done as long as you have one server in your environment running SQL 2005 still.
You just need to create a linked server for your 2008 servers. Then, in SSRS, you create one data source to your 2005 server, but reference the 2008 linked server in your query, and it'll work. For example, if you have a SQL 2008 server called Chicago and a SQL 2005 server called Detroit, you'd create a linked server on Detroit for Chicago, and you'd create a data source in Visual Studio 2005 for your report that would point to Detroit. Then, in your query, you'd just have something like SELECT * FROM Chicago.Database.dbo.Table and it should work.
Obviously you'll probably want to switch to a 2008 environment altogether, but this is a nice thing to know for the conversion phase.
Hope that helps.