we are using extensively the silverlight telerik controls in our application and we are currently facing big problems regarding rendering on mac. However, we are not sure that it is an issue with the telerik controls. We have posted a description of the problem in several places already, so I will just link to the silverlight forum post and hope that you have seen this before to maybe give us some hints.
We have tried with the silverlight 5 beta plugins on the client, and this bug seems to disappear. Also, this bug doesn't seem to occur on every mac hardware (we could not reproduce it on imacs with older version of macos, but it was consistent on macbooks with snow leopard)