The problem comes from the fact that when using DNN your page is in quirksmode. The RadControls are XHTML compliant and in order to make them work correctly you should have the corresponding <!DOCTYPE> declaration on your page. Unfortunately, when using DNN you cannot do this.
We improved the behavior of some controls as the RadToolTip when they are in quirksmode because of our customers who use DNN. However, this task is hard and time-taking. I logged the reported problem in our database but I cannot tell you when exactly it will be fixed.
the Telerik team