Starting from SQL Server 2008 SP1 Report Builder 2.0 can now be deployed as a click-once application. However this is not the default behavior after you install SP1.
If you click the "Report Builder" icon in the Report Manager, the old Report Builder will pop up.
To change this go to the Site Settings and change the URL under "Custom Report Builder launch URL" to following:
This should do the trick. Have fun with Report Builder 2.0