If you need to use the newest RadControls, you could always swap the Telerik.Web.UI.dll
file stored in the bin folder with the one with version 2008.1.415.20.
In this way you will be able to run the latest available version of the Telerik's RadControls. It would be necessary to add assembly binding in the web.config so that you can make the swap legal. The highlighted part from the snippet is the one which should be added:
| <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
| <assemblyIdentity name="Telerik.Web.UI"
| culture="neutral" />
| <bindingRedirect oldVersion="2007.3.1425.20"
Nevertheless, we recommend you to wait until Sitefinity Service Pack 2 release is available. It will include the newest versions of RadControls that are not released yet.
Hope that the information is helpful.
All the best,
the Telerik team