I'm using the latest RIA Services Beta and I have not seen any issue with the Q3 Telerik controls. Now I only upgraded a few days ago and I haven't used all the Telerik controls in my application, but the radgrid control seems to work as before. I'm doing all the manipulation of the DataContext myself in code behind and I'm not using the DomainDataSource. I'd be interested if Telerik can comment on whether they think the latest RIA services beta should cause issues with the Q3 stuff.
Now there are a few breaking changes in RIA which you'll have to work through. Plus there is a significant BUG which prevents the deployment/publishing onto a hosted site (works fine when running locally in the dev environment). There is a workaround out on the ria services forum and msoft will be issuing an update soon.
You can find the thread about the issue here
and the fix summary is here